Stowe Final Report, March 2020

Surficial Geology and Groundwater Hydrology of the

Stowe7.5 Minute Quadrangle, Vermont

Stephen F. Wright

Department of Geology, University of Vermont

March 2020

Table of Contents

Executive Summary/Significant Findings ......................................................................................................... 2

Introduction, Location and Geologic Setting, ................................................................................................... 3

Prior Work ......................................................................................................................................................... 5

Methods ............................................................................................................................................................ 6

Stowe Surficial Geologic Map .......................................................................................................................... 7

Surficial Geologic Mapping Units ..................................................................................................................... 7

Geologic Cross-sections................................................................................................................................. 15

Isopach Map of Surficial Materials ................................................................................................................. 16

Surficial Geologic History of the Stowe Quadrangle ...................................................................................... 16

Groundwater Hydrology of the Richmond Quadrangle .................................................................................. 18

Water Table Contour Map with Flow Lines .................................................................................................... 18

Recharge Potential to Surficial Aquifers Map ................................................................................................. 19

Bedrock Hydrologic Units Map ....................................................................................................................... 20

References ..................................................................................................................................................... 20

Vermont Geological Survey Contract # 38392

Figure 1: Glacially striated erratic perched on a larger erratic on land owned by the Green Mountain Club between Stowe Village

and Waterbury Center.

Vermont Geological Survey Open File Report VG2020-1

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 2

Executive Summary/Significant Findings

The surficial geology of the Stowe 7.5-Minute Quadrangle was mapped during the summer of 2019 with field

assistance from four geology students at the University of Vermont. Considerable detail has been added to prior map

created by (Stewart and MacClintock, 1970). A surficial geologic map, four geologic cross-sections, an overburden

thickness map, potentiometric surface map, recharge potential to surficial materials map, and a bedrock hydrologic

unit map are included with this report.

The Stowe Quadrangle contains a variety of glacial landforms and sediments that formed as the Laurentide ice sheet

first flowed across the Green Mountains and then thinned and retreated from the area. Glacial striations are almost

universally oriented NW-SE. The asymmetry of roches moutonnée (smooth abraded side to the NW, quarried side to

the SE), glacial erratics sourced from the northwest, and small-scale stoss and lee bed-forms on outcrops all indicate

that ice flow was to the southeast when the ice sheet was sufficiently thick to cover the mountains. When the ice

sheet thinned, its flow was guided by topography, from north to south in the Little River valley.

Till mantles all of the upland areas. Most is dense lodgment

ll, but some may be till remobilized as debris flows

sourced from the steep mountain hillsides. Most of the till cover is thin, but extensive areas exist where the till is thick

enough to completely mask the underlying bedrock topography. The surface till in these areas may overlie older

glacial deposits consisting of till or glaciolacustrine sediments.

During ice retreat ice-contact sediments, originating in subglacial tunnels or in subaqueous fans, were deposited in

some of the valleys. These are only evident via water well logs where they occur at the bottom of deep valleys and

are overlain by younger glaciolacustrine sediments. Nevertheless, they comprise a significant aquifer confined by and

protected by those same fine-grained glacial lake sediments.

The retreating ice sheet dammed two different glacial lakes that filled the Little River valley to different elevations.

Glacial Lake Winooski formed first and grew as the ice retreated west and north across the area. With the onset of

the Older Dryas cold period, ~14,100 years ago, the ice readvanced down the Little River valley and then up the

Winooski River valley. This readvance of the ice sheet deposited a second “readvance

till” ac

ross some parts of the

quadrangle and formed at least one moraine that was recognized while mapping in the quadrangle. At the end of the

Older Dryas the ice sheet began retreating again and Glacial Lake Winooski once again grew and eventually

occupied most low-lying areas east of the mountains. Glacial Lake Winooski partially drained when the ice sheet

retreated as far west as Jonesville creating another lower-elevation lake, Glacial Lake Mansfield. Deltas that formed

in both these lakes occur within the quadrangle, however the majority of the glacial lake sediments consist of medium

to fine sand, silt, and clay that accumulated in the deeper parts of these lakes, i.e. the Little River valley and its

tributaries.

Well data was utilized to contour the overburden thickness. Similar to most valleys in Vermont, thick sections of

surficial materials accumulated in the valleys whereas the high and steep mountain slopes are overlain by a

discontinuous cover of till. An old buried channel of the Little River was discovered in the valley parallel to Barrows

Road. The current channel of the river is relatively new which is why the bedrock that underlies the

terfall between

Stowe’s upper and lower villages still exists. This bedrock lip has severely limited upstream erosion by either the

West Branch or the Little River. As a result, these rivers have meandered back and forth across these broad, flat

valleys that are still largely filled with glacial lake sediment, but have not eroded deeply into them. These areas have

historically provided some of the best agricultural land in the area.

The abundance of coarse grained surficial materials in many of the valleys constitute both good areas for

groundwater recharge and good surficial aquifers. In areas where ice-contact sediments are overlain by thick sections

of fine-grained lacustrine sediments, these become nicely protected confined aquifers. No systematic association

exists between well yields and the mapped bedrock units within the quadrangle. This is because all these units are

metamorphic rocks with no primary porosity. Groundwater accessed by wells within the quadrangle occurs within the

secondary porosity of these rocks, the fractures.

The water table across the map largely mimics the ground surface topography. Large-scale groundwater flow is

towards the major rivers that drain the area: the Little River and its tributaries.

search supported by the U. S. Geological Survey, National Cooperative Geologic Mapping Program, under USGS

award number G19AC00158. The views and conclusions contained in this document are those of the authors and

should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U. S.

Government.

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 3

Introduction

This report describes the results of mapping surficial geologic materials and landforms at a scale of 1:24,000 in the

Stowe 7.5-minute Quadrangle, northern Vermont during the summer of 2019. Numerous observations have been

used to create a geologic map of the area and these observations underlie the interpretations presented here. This

report also describes several derivative maps generated for this project that focus on the area’s groundwater

hydrology. A major objective of this work was to describe the three-dimensional distribution of surficial materials in the

mapped area. These surficial materials, in addition to measurements of glacial striations, provide the basis for an

interpretation of the different environments that existed as the Laurentide Ice Sheet was flowing and subsequently

retreating across this area. Other landforms and associated surficial materials offer insight into processes occurring

during the Holocene well after the ice sheet retreated.

Location and Geologic Setting

Much of the Stowe Quadrangle lies in a broad valley between the Green

Mountains to the west and the Worcester Range to the east, but also

includes a portion of the high Worcester Range ridge in the southeastern

corner (Fig. 1). The south-flowing Little River and its tributaries drain

most of the area which flows into the Winooski River a short distance

south of the quadrangle. The bedrock geology of the area is summarized

on the Vermont Bedrock Geologic Map (Ratcliffe et al., 2011). Rocks

underlying this area consist of metasedimentary rocks (schist and

phyllite) that were originally deposited as sediments in the Iapetus ocean

along the margin of Laurentia from late Precambrian through

early/middle Ordovician time. Locally, mafic igneous rocks intruded

these sediments or erupted as lava flows or pyroclastic deposits across

the ocean floor. All of these rocks were subsequently deformed and

metamorphosed during the Taconic Orogeny and again during the

Acadian Orogeny. Rock units in this area are typically bounded by

generally north-south striking thrust faults and lesser normal faults

occurring on a wide range of scales that generally mimic the north-south

trend of the mountain belt (Ratcliffe et al., 2011).

The surficial geologic materials occurring in the region were

predominantly deposited during the most recent (Wisconsinan) glaciation

in glacial or periglacial environments existing during or shortly after the

Laurentide ice sheet retreated across this area ~14,000–13,500 years

ago (Corbett et al., 2019; Ridge et al., 2012). A variety of surface

processes have redistributed these materials during the Holocene. The

Surficial Geologic Map of Vermont (Stewart and MacClintock, 1970) shows the general distribution of these

sediments based on reconnaissance mapping completed during the 1960’s (Fig. 2).

Figure 1: Shaded-relief map of

Vermont showing the location of the

Stowe7.5-minute quadrangle (red

rectangle) situated immediately east of

the Green Mountains.

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 4

The ice sheet in northern New England was sufficiently thick to completely cover the mountains. Consequently, the

mountains are largely mantled by till with isolated pockets of organics occurring in wetland areas. Ice-contact

sediments, deposited beneath or adjacent to the retreating ice sheet, are found in many of the major river valleys.

These ice-contact sediments are largely overlain by glaciolacustrine sediments deposited in several ice-dammed

glacial lakes that flooded the valley (Larsen, 1972, 1987; Wright, 2018a). The largest of these was Glacial Lake

Winooski which formed when the retreating ice sheet dammed the Winooski River (Fig. 3A). A lower elevation lake,

Glacial Lake Mansfield, formed when the retreating ice sheet uncovered a lower outlet (Fig. 3B).

Figure 2: Map shows the distribution of surficial materials within the Stowe Quadrangle based on

reconnaissance mapping utilized to complete the Vermont Surficial Geologic Map (Stewart and MacClintock,

1970).

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 5

In addition to extensive deposits of fine-grained sediments deposited in the deeper waters of these lakes, deltas

formed where streams entered these glacial lakes. After these glacial lakes drained these same streams have eroded

channels through the glaciolacustrine sediments during the Holocene leaving abandoned fluvial terraces along these

valleys. Further stream erosion in the uplands has led to the deposition of numerous alluvial fans at the base of steep

slopes.

Prior Work

The Stowe 7.5-minute quadrangle is the NW quadrant of the Montpelier 15-minute quadrangle which was mapped in

reconnaissance-fashion by Stewart (1956–1966) using 1:62,500-scale topographic base maps. These open-file maps

were incorporated into the Surficial Geologic Map of Vermont which is the source for the map presented in Figure 2

(Stewart and MacClintock, 1970). Springston and Dunn (2006) mapped a portion of the southern quadrangle

encompassing the drainage basins of Bryant and Thatcher Brooks as part of a larger project mapping the southern

Worcester Mountains watershed. Wright (2018b) mapped the western side of the Little River valley north of the

Waterbury Reservoir when mapping the Bolton Mountain Quadrangle, the quadrangle lying immediately west of the

Stowe quadrangle. Wright (2015b) has interpreted the ice flow history across this part of northern Vermont from a

large compilation of glacial striation azimuths and has modeled the readvance of the Laurentide ice sheet in the

adjacent Winooski River valley (the Middlesex Readvance) during the Older Dryas (Wright, 2015a).

Glacial

Lake

Wiinooski

Glacial

Lake

Mansfield

Figure 3: (A) Extent of Glacial Lake Winooski in the Stowe Quadrangle (lake elevation ~323 m). (B) Extent of Glacial

Lake Mansfield in the Stowe Quadrangle (lake elevation ~235 m). The Worcester Range is the prominent ridge in the

southeastern corner of the quadrangle.

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 6

Methods

Surficial geologic mapping of the Stowe Quadrangle was completed during the summer 2019 field season. Field

observations were recorded using a Fulcrum data collection App modified for surficial field mapping. During this time

over 3,000 separate field observations were recorded using the (1) shaded-relief LiDAR imagery, (2) topographic

map, and (3) aerial photography as base maps. The locations of these observations are shown on Figure 4.

Additional observations along the west side of the Little River valley immediately north of the Waterbury Reservoir

were collected during an earlier field season (Wright, 2018b). A shape file detailing the location and geological

observations recorded at each site was generated by the Fulcrum mapping app. These field data, the LiDAR imagery,

and topographic contour lines generated from the LiDAR DEM’s provide the basis for generating the surficial geologic

map that accompanies this report.

Figure 4: Red dots show locations of field observations within the Stowe and adjacent quadrangles.

Stowe Quadrangle

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 7

Four UVM undergraduate students, Caleb Bogin, Sarah Powers, Abby Zani, and Corey Beutel, assisted with field

work during 4 weeks in June. The students also assisted with 2 days of field work on the Cotton Brook landslide, a

large landslide that occurred during May of 2019 just a few kilometers west of the Stowe Quadrangle boundary

(Springston et al., 2020). When the weather was unsuitable for mapping students were introduced to GIS mapping in

the UVM Geology Department’s computer lab and began digitizing their data. Student mapping included most areas

within the quadrangle west of Route 100 south of Stowe village and then west of a north-south line extending north

from Stowe village. Students also collected outcrop locations and glacial striations along the trail up Stowe Pinnacle

and the trail between Stowe Pinnacle and Mount Hunger/White Rocks. The students presented their map, cross-

sections, and interpretations on a poster (Bogin et al., 2020). While the combined Northeast/Southeast Geological

Society of America meeting in Reston, Virginia was canceled because of the Corona virus, the students’ poster will

be eventually be accessible via the Vermont Geological Society website.

Only limited field work was conducted in the Thatcher Brook valley as this area had been recently mapped

(Springston and Dunn, 2006). Consequently, geologic contacts and materials mapped in this area broadly follow

those on the earlier map, but have been modified in some areas based on the stratigraphy described in water well

logs and limited field observations.

Stowe Surficial Geologic Map

The surficial geologic map that accompanies this report shows the aerial distribution of different types of surficial

materials, landforms constructed of these materials, glacial striations, large erratics, and water wells. During the

spring of 2018 the Vermont Geological Survey developed a uniform set of mapping units which are utilized on the

Stowe Surficial Geologic Map (Springston et al., 2018). The boundaries between these different materials are

geologic contacts and are shown as solid lines on the geologic map. It’s important to realize, however, that these

contacts are non-planar 2-D surfaces that extend out-of-sight below Earth’s surface and their extension above Earth’s

surface has eroded away. In some areas geologic contacts could be closely located in the field and these locations

were recorded and used when

constructing the map. However, in most areas the location of these contacts is

interpreted from field observations, distinctive landforms, and aerial imagery. Every effort was made to make these

contacts as accurate as possible, but there is an element of interpretation in the placement of these contacts.

Surficial Geologic Mapping Units

The different surficial materials mapped within quadrangle are described below, in stratigraphic order, from oldest to

youngest. Most of the surficial materials in the area were deposited by or adjacent to the Laurentide ice sheet as it

flowed across and then gradually thinned and retreated across the area.

Bedrock Outcrops/Glacial Striations and Grooves

Bedrock outcrops were mapped when they were

encountered during field traverses. Additionally, most

outcrops occurring along town roads and state

highways were also mapped. No attempt was made

to map all outcrops, especially in the upland areas

where outcrops are numerous and closely spaced.

Some of the mapped outcrops still retain either or

both glacial striations or grooves produced by glacial

abrasion. Where visible, the orientations of linear

features were recorded. In northern Vermont roches

moutonnée, erratics derived from sources northwest

of erratic locations, and tails of uneroded rock

southeast of quartz veins all indicate that the ice

sheet flowed from northwest to southeast across the

area (Wright, 2015b). Consequently, arrows used on

Figure 5: Grooved bedrock outcrop on the east side of

Mount Hunger.

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 8

the geologic map to show the orientation of glacial striations and grooves point generally southeast and south in the

inferred down-ice flow direction.

Glacial Till

Glacial till directly overlies the bedrock in most areas. Within the quadrangle, till is the ubiquitous surficial material on

the ground surface in areas above the valley bottoms. The freshest exposures are produced by stream erosion and

also appear in landslides where the till is gray to light brown and very dense (Fig. 6). Till in the area consists of

angular to subrounded pebbles, cobbles, and boulders, many with striated surfaces) suspended in a fine

clay/silt/sand matrix (Fig. 3A). Most of the till occurring in this area is lodgement till consisting of materials eroded,

deformed, and deposited beneath the ice sheet. Close to the ground surface frost heaving, plant roots, and animal

borrows have loosened the till and surface run off has eroded some of the smaller-sized sediment (the “fines”) in the

till. Ablation till was undoubtedly at least locally deposited on top of the lodgment till, but its texture is similar to

lodgment till loosened by frost heaving etc. No attempt was made to systematically measure the composition of the till

by either grain size or composition nor were any till fabric measurements made.

Figure 6: Dense, grey glacial till exposed in a small stream-cut along an empheral tributary to Miller Brook. Most

clasts in the till are small, but large boulders are not uncommon.

The thickness of till in the upland areas of the quadrangle varies considerably. Generally, the till cover is thin (less

than 2 to 3 meters) and abundant outcrops are present. However, the till across extensive areas in the upland valleys

is sufficiently thick to completely mask the underlying bedrock. No wells exist in these areas, but tributary streams

frequently incise through more than 10 m of till. Large (>4 m diameter) erratics, composed of locally derived

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 9

metamorphic rocks, are common and were mapped where encountered (Fig. 7).

Figure 7: Exceptionally large erratic along a small tributary east of the Waterbury Reservoir. Erratic diameter exceeds 15 m.

Readvance Till

A stratigraphically continuous, albeit thin layer of diamict was mapped by Springston and Dunn (2006) across a high-

elevation terrace in the Thatcher Brook valley. This diamict overlies glaciolacustrine sediments and was interpreted to

be a readvance till deposited when the ice sheet readvanced across a recently deglaciated landscape in response to

climatic cooling, the Middlesex Readvance (Wright, 2015a). A cross-section illustrates the geometry of the readvance

till and other glacial materials in this area (Fig. 8). Another diamict comprised largely of structureless silt/sand mixed

with rocks was mapped southwest of Moss Glen Brook in the northeastern corner of the quadrangle where it occurs

in a landform interpreted to be a moraine. This material is similarly interpreted as a readvance till produced as the ice

sheet advanced across recently deposited glaciolacustrine sediments and pushed these materials into a moraine.

Stewart and MacClintock (1970) mapped this moraine, but show it extending far beyond what is shown on the current

map (Fig. 2).

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 10

Figure 8: Cross-section C-C' showing the vertical distribution of glacial sediments in the Thatcher Brook drainage. Well logs

document the thin readvance till (Ptr) overlying a thick section of fine-grained glaciolacustrine sediment (Plf: silt/fine sand). The

lacustrine sediments overlie the widely distributed basal till (Pt) that extends across Barnes Hill and the highlands to the

southeast. See geologic map for location of section.

Ice-Contact Deposits

Ice-contact deposits largely consist of fluvial (stream-deposited) sediments deposited under, adjacent to, or in front of

the retreating ice sheet. These streams are generally fast-moving and therefore carry and deposit coarse-grained

sediment, dominantly sand and gravel. Sediments deposited in subglacial tunnels form ridges of sand and gravel

(eskers) once the glacier melts away. No ice-contact sediments were mapped in the Stowe Quadrangle. However,

well logs indicate that coarse-grained sediments underlie fine-grained glaciolacustrine sediments in the Little River

valley north of Stowe Village (Fig. 9). A cross-section (B-B’) illustrates the vertical distribution of sediments in the

valley. Well 400 taps a confined sand and gravel aquifer here interpreted as an esker although this could also be part

of a subaqueous fan deposited near the mouth of an esker tunnel.

Figure 9: North-south cross-section across the Little River valley north of Stowe village (see geologic map for location). Thin

alluvial terrace (Hat) and alluvim (Ha) deposits overlie a thick section of fine-grained glaciolacustrine sediments (Plf). These

fine-grained sediments overlie coarser, more permeable sediments that serve as the aquifer that

feeds wells 582 and 400.

Lacustrine (Glacial Lake) Deposits

Glacial lake deposits include both coarser-grained sediments deposited at or near the shorelines of lakes (deltas and

beaches) and finer-grained sediments deposited in the deeper quiet-water parts of lakes (Fig. 10). Fine-grained

lacustrine sediments are common in most of the valleys in the quadrangle (see Plf unit on the geologic map and in

Figs. 8 and 9) and several deltas have also been mapped. Larsen (1972, 1987) outlined the sequence of glacial lakes

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 11

that occupied the Winooski River drainage basin as the retreating ice sheet dammed the west-flowing river preventing

it from draining into the Champlain valley. The highest regional lake, Glacial Lake Winooski, extended across the

Little River valley as well as tributary valleys in the quadrangle (Fig. 3). The geologic map traces a projection of the

shoreline of this lake across the quadrangle

. As the ice sheet retreated down the Winooski River valley it uncovered

lower outlets allowing Glacial Lake Winooski to partially drain through the Huntington River valley and Hollow Brook

into the Champlain Valley. The lower-elevation regional lake that formed is called Glacial Lake Mansfield and the

designators 1 and 2 are used to refer to two outlets of that lake at slightly different elevations (Fig. 3; Larsen, 1972,

1987). The geologic map also includes the projection of the shoreline of this lake at its higher elevation, i.e. Glacial

Lake Mansfield 1. A stratigraphic section exposed along the Waterbury Reservoir in Little River State Park records

the transition from Glacial Lake Winooski to Glacial Lake Mansfield 1 (Larsen et al., 2003). Correlation with the well-

dated Connecticut Valley varve sequence allows this drainage event to be dated to ~13,800 years ago (Ridge et al.,

2012; Wright, 2018a). Glacial Lake Winooski varve counts indicate that this lake evolved over a time span of less

than 300 years and likely only existed in the Stowe Quadrangle area for less than 200 years. Glacial Lake Mansfield

lasted for a much shorter period

of time as the ice sheet needed to retreat only a short distance for this lake to merge

with Glacial Lake Vermont, a lake too low in elevation to flood any of the valleys in the Stowe Quadrangle (Wright,

2018a).

Several glacial lake deltas were mapped in the area. Significant areas of sand and gravel were deposited as deltas in

Glacial Lake Winooski in (1) the northwest corner of the quadrangle where the West Branch of the Little River flowed

into the lake (the bulk of this delta lies near Stowe Fork), (2) in the northeast corner of the quadrangle where Moss

Glen Brook entered the lake, and (3) in Stowe Hollow where Gold Brook entered the lake. Similarly, streams flowing

into the lower-elevation Glacial Lake Mansfield also deposited deltas. The best preserved of these lies northeast of

the intersection of Gold Brook Road and Route 100. The broad terrace that Stowe High School is built on may also

The shoreline projection of both Glacial Lake Winooski and Glacial Lake Mansfield 1 are from work by George

Springston. Calibration, based on recent detailed mapping, is ongoing.

Figure 10: Fine-grained glaciolacustrine sediments (Plf) exposed (A) near the north shore of the Waterbury Reservoir and (B) in

Stowe Hollow. Sediments in both photos consist of yearly layers of silt and clay (varves). Soft-sediment deformation is evident near

the shovel blade and resulted when weak unconsolidated sediments flowed in response to a density inversion (sand over water-

saturated silt) or slumping along a sloping lake-bottom surface.

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 12

be a Glacial Lake Mansfield delta. A third Glacial Lake Mansfield delta occurs where Moss Glen Brook partially

eroded its older Glacial Lake Winooski delta and redeposited these sediments in both a fan (see below) and a delta.

Alluvial Fans (Pleistocene and Holocene)

Alluvial fans are fan-shaped deposits formed where steeply-sloping streams deposit sediment where they flow out

onto a gently-sloping valley floor, e.g. a fluvial terrace or modern floodplain. Sediments deposited in alluvial fans

generally grade from coarse to fine between the apex of the fan to its toe. The absolute size range of sediment in

fans depends on the source of sediment. In many upland areas fans are sourced from till remobilized as debris flows

and the fans consist largely of unsorted diamict. In areas where streams are eroding channels through fine-grained

lacustrine sediment, that will be the size of sediment deposited in the fan.

Alluvial fans of different sizes and age are shown on the geologic map. Pleistocene alluvial fans have been mapped

in areas where Glacial Lake Winooski deltas were incised by streams immediately following their abandonment when

the lake partially drained forming Glacial Lake Mansfield. The eroded delta sediments were deposited as fans that

graded into Glacial Lake Mansfield deltas.

Holocene fans are also common in the area. Studies in northern Vermont indicate that alluvial fans have been active

episodically throughout the Holocene and have many received

their most recent pulse of sediment following

European land clearing in the late 18th and early 19th centuries (Bierman et al., 1997; Jennings et al., 2003). Related

work by Noren et al. (2002) recording pulses of clastic sediment deposited in ponds and small lakes, indicates that

pre-European settlement erosion has not been uniformly distributed throughout the Holocene and seems instead to

be concentrated during periods of increased high-intensity storms. If climate shifts produce a greater frequency of

high-intensity storms, further sedimentation on the area’s alluvial fans seems likely.

Fluvial Terrace Deposits

Fluvial terrace deposits are stream

sediments (alluvium) occurring on

terraces well above modern streams

(Fig. 11). Most commonly these

sediments were deposited when

streams began flowing across and

incising into sediment-filled valleys

after the glacial lakes drained. As

streams eroded channels more and

more deeply through earlier-

deposited sediments, older channels

were abandoned. Consequently, in

many areas mapped as fluvial terrace

deposits there are several terrace

levels and old abandoned channels

are visible in many of these terraces.

In most areas these fluvial terraces

are underlain by a veneer of sand

and gravel corresponding in

thickness to the depth of the stream

channel that deposited the sediment.

In some areas gravel on these terraces has been partially or largely mined away revealing the underlying sediments.

For geological consistency, terraces where the alluvium has been mined were still mapped as fluvial terrace deposits.

Figure 11: Abandoned fluvial terrace (Hat) along River Road, west side of Little

River valley. Meadow is underlain by sand and gravel deposited by the Little River

before it eroded ~10 m down to its current level to the right of the photograph.

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 13

Alluvium

Alluvium refers to sediments deposited

by modern rivers and streams. Alluvium

includes sand and gravel deposited in

river channels and point bars, but also

includes sand and silt deposited on

floodplains that are frequently forested

or farmed (Fig. 12). Organic materials

are a frequent component of modern

alluvium. Broad areas of alluvium lie

adjacent to the Little River and its

tributaries.

As soon as the glacial lakes that once

occupied the valleys drained, streams

began flowing across the sediment

deposited in these lakes. In most areas

at least some of the underlying glacial

lake sediments were eroded before

alluvium was deposited. Figure 13 shows

a rare exposure of Little River alluvium in

contact with the underlying glacial lake

sediments. In smaller upland stream

valleys, alluvium commonly is deposited

directly on glacial till.

In many if not most areas modern

alluvium is in contact with fluvial terrace

deposits (old alluvium). The contact

between these two mapping units is an

interpretation of how much area is

flooded during high-water events. The

thickness of alluvium is proportional to

the size of the stream that deposited it,

but generally corresponds to the depth of

the

modern stream channel.

North of the confluence of the West

Branch and the Little River the Little

River valley becomes very broad where

it’s joined by Sterling Brook (Fig. 14). This

wide area has almost no relief and is most

likely the little-eroded bottom of Glacial

Lakes Winooski and Mansfield mantled

with a thin veneer of alluvium and alluvial terrace deposits. The alluvium across much of this area dominantly consists

of fine-grained sediment as that was the material Sterling Brook and the Little River were eroding and transporting in

this area. A diffuse drainage divide crosses this area separatig water flowing north into the Lamoille River from water

flowing south into the Winooski River (via the Little River). Several large kettles occur in this area indicating that

blocks of the ice sheet detached where the ice sheet thinned over this drainage divide. They were likely quickly

buried by ice-contact sediment emanating from one or more subglacial tunnels in the ice sheet and/or sediment

Figure 12: Alluvium exposed along a reach of the Little River lying between

River Road and Route 100. Pebble/cobble gravel is exposed on the bar at right

whereas finer-grained sand/silt/organics are exposed in the cutbank at left.

Figure 13: Approximately 2 m of alluvium is deposited directly on fine-grained

glacial lake sediments exposed near the waterline. When the last glacial lake

drained from the valley the Little River began flowing across and eroding the

recently deposited lake sediments. Mayo Farm property, Stowe.

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 14

deposited by the Moss Glen Brook Delta.

Figure 14: View looking south across a broad area of alluvial terrace deposits (Hat) lying between the Little River and Sterling

Brook. A thin veneer of alluvium overlies fine-grained glacial lake sediments deposited on the bottom of both Glacial Lakes

Winooski and Mansfield.

Swamp/Wetlands

Wetland areas generally occupy closed basins and

display varying amounts of open water depending on the

season and the water table elevation (Fig. 15). The

dominant surficial material in wetland areas consists of

both living and partially decayed organic materials but

also includes inorganic clastic sediment washed into

these areas by streams and overland flow. The borders of

wetland polygons were mapped using satellite imagery,

but the boundaries shown on the geologic map should

not be used for regulatory purposes.

Artificial Fill

Areas mapped as artificial fill often consist of materials

used to fill stream valleys crossed by roads but also

include materials used for construction where the water

table is close to the surface. Fill is shown on the geologic

Figure 15: Wetland area near northern boundary of the

quadrangle. Sediments accumulating here consist of both

organics and silt/sand washed in from the surrounding area.

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 15

map where it could be mapped using the LiDAR imagery. Materials comprising fill are generally sand and gravel, but

can also include broken and crushed rock, concrete, and other waste materials.

Geologic Cross-Sections

Four geologic cross-sections were constructed for this project. All of them are included on the geologic map sheet

where their locations are also shown. Two were described earlier in this report and the remaining two are described

below. Cross-sections present an interpretation of the different surficial materials lying beneath Earth’s surface and

their thickness. The best information available about the type and thickness of surficial materials in most areas comes

from the logs kept by drillers when completing domestic water wells. The most accurate information recorded by

drillers is the “depth to bedrock” or “overburden thickness” as this is approximately equivalent to the length of steel

casing needed for a drilled well, a length carefully recorded and charged to the home owner. Otherwise, the quality of

records kept by drillers of the type and thickness of the surficial material they drill through varies enormously and

frequently requires considerable interpretation when constructing a cross-

section. Many well logs are so poor that

they provide no useful geological information. Geologic cross-sections were generally drawn where a reasonable

number of water wells with good logs were aligned across major valleys. All are drawn with a 2.5x vertical

exaggeration making slopes appear steeper than they actually are and units thicker.

The Little River Farm Road Cross-section (A-A’, Fig. 16) displays a distribution of surficial materials that is common in

the region. Glacial till (Pt) mantles the steeper slopes above the valley whereas most of the materials filling the valley

consist of fine-grained lake sediment (Plf). The silt/clay was most likely deposited when Glacial Lake Winooski

occupied the valley whereas the sand was likely deposited in the much shallower Glacial Lake Mansfield. These lake-

bottom sediments were eroded into a series of terraces by the Little River once the glacial lakes drained. Each

terrace is underlain by river alluvium (Hat) including those adjacent to the modern river channel (Ha).

Figure 16: Approximately West-East geologic cross-section across the Little River valley northeast of Stowe Village. Most

surficial sediment occurring in

the valley consists of fine sand, silt, and clay deposited in the relatively deep parts of Glacial

Lakes Winooski and Mansfield. Terraces on either side of the river are underlain by “old” alluvium deposited when the Little

River was flowing across the underlying lake-bottom sediments.

The Waterbury Center Cross-section (D-D’, Fig. 17) extends almost north-south through Waterbury Center and

across the Bryant Brook valley not far upstream from where it enters the Waterbury Reservoir. Numerous wells

indicate that the valley is filled with fine-grained glacial lake sediment. The terrace which the village of Waterbury

Center is built on is underlain by sand. This is interpreted to be sand also deposited in a glacial lake (Glacial Lake

Mansfield) but it may be sand that was eroded from farther upstream and deposited as the fine-grained toe of an

alluvial fan. Coarser grained sediments penetrated by some wells are interpreted to be ice-contact sediments (Pic).

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 16

Figure 17: Cross-section through Waterbury Center and Bryant Brook (view looks west). The bedrock valley is well constrained

by water wells and is filled with both coarser ice-contact sediments (Pic) and fine-grained glacial lake sediments (Plf). Terraces

mark former positions of Bryant Brook as it has eroded through the glacial lake sediments.

Isopach Map of Surficial Materials

The “Isopach Map of Surficial Materials” contours the thickness of surficial materials (overburden) within the

quadrangle. The data used to generate this map are (1) bedrock outcrops recorded from this study (which indicate

areas where surficial materials don’t occur), (2) bedrock outcrops visible on the LiDAR shaded-relief imagery, and (3)

records of overburden thickness from domestic water wells. The well locations were not checked and errors, some

significant misplacements of wells, occur which affect the accuracy of these contours. These data are contoured

using a 20-foot contour interval between 0 and 100 feet and 100-foot contours in areas where surficial materials

exceed 100 feet. Contouring algorithms applied to the overburden thickness data produced geologically unrealistic

contours, so these data were contoured by hand. In

general, isolated wells reporting thick surficial materials were

ignored, i.e. bullseyes were not drawn around these isolated wells.

Generally, areas of thick surficial materials occur in the major river valleys (see the geologic cross-sections described

earlier in this report). The valleys have accumulated not only the till that’s also present in the uplands, but all of the

ice-contact (e.g. esker) and glacial lake sediments that accumulated as the ice sheet was retreating across this area.

In many parts of these valleys sediment fill exceeds 100 to 200 feet indicating that preglacial erosion was substantial

and in many areas these preglacial valleys are largely hidden by the glacial sediments that currently fill them. One

striking example is the thick accumulation of sediment lying beneath the terrace that Stowe High School is built on. A

deep buried bedrock channel extends south from here to the modern channel of the Little River. This is the paleo-

channel of the Little River and its current channel across the waterfall between the lower and upper village of Stowe

is new (see discussion in below section). The bedrock channel beneath the Little Ri

ver valley north of Stowe Village

is also deep and may indicate a preglacial channel that extended north into the Lamoille River valley.

Surficial Geological History of the Stowe Quadrangle Area

The observations gathered during this mapping project combined with many other studies provide the basis for

interpreting this area’s surficial geologic history. The highest mountains in the area are marked by glacial striations

and contain remnant patches of glacial till. These observations indicate that the last ice sheet completely covered the

mountains. Modeling by the author suggests that at the peak of glaciation, ~25,000 years ago, the ice surface over

the northern Green Mountains was at an elevation of almost 2,200 m, i.e. ~840 m of ice (~2,750 ft) covered

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 17

Vermont’s highest peak, Mount Mansfield. Glacial striations across both the Green Mountains and the Worcester

Range are oriented NW-SE indicating that the ice sheet was flowing to the southeast, obliquely across the mountains

and intervening valleys (Wright, 2015b). As the ice sheet thinned its flow direction became topographically controlled

by the orientation of the large valley lying between the Green Mountains and the Worcester Range, the valley which

occupies most of the Stowe Quadrangle. Glacial striations in the valley are dominantly oriented parallel to it and

formed when the thinning ice sheet was flowing approximately south.

As the ice sheet was retreating, most meltwater generated at its surface flowed through the ice to its base where it

collected in large subglacial tunnels. Sediments eroded from the underlying till as well as from the ice were carried in

subglacial streams and eventually discharged at the margin of the ice sheet. As noted earlier, no ice-contact

sediments were mapped at the surface of the Stowe Quadrangle, but coarse sand and gravel encountered at depth in

drill cores are interpreted to be ice-contact sediments (Figs. 9, 17).

When the margin of the ice sheet retreated across north-central Vermont the drainage divide separating water flowing

east and south into the Connecticut River from water flowing north and west into Lake Champlain was uncovered. As

the ice sheet retreated north of this divide, water that would normally flow north and west into the Winooski River

instead pooled up forming a glacial lake, Glacial Lake Winooski (Larsen, 1972, 1987). This lake grew in size as the

ice sheet retreated and eventually covered all of the Winooski and Lamoille drainage basins east of the Green

Mountains (Fig. 3A; Wright, 2018a). However, there is good evidence that while Glacial Lake Winooski was

expanding northward, the ice sheet reversed its retreat and began advancing back across areas it had recently

uncovered. This event is locally referred to as the Middlesex Readvance and likely occurred during a ~100-year-long

cooling period referred to as the Older Dryas, ~14,100 to 14,000 years ago (Ridge et al., 2012; Wright, 2015a).

During this time a thin layer of till was deposited on top of sediments recently deposited in Glacial Lake Winooski and

locally moraines formed

along the ice margin (See northeast corner of the geologic map; Springston and Dunn,

2006). The extent of this readvance till indicates that the ice front advanced back up the Winooski River valley well

beyond Montpelier before it withdrew again allowing Glacial Lake Winooski to regrow (Wright, 2015a).

After the Middlesex Readvance, Glacial Lake Winooski lasted for less than 200 years. As the ice sheet retreated

down the Winooski River valley it uncovered lower outlets allowing Glacial Lake Winooski to partially drain, albeit

catastrophically, through the Huntington River valley and Hollow Brook into the Champlain Valley. This transition can

be accurately dated and occurred 13,820 years ago. The lower-elevation regional lake that formed is called Glacial

Lake Mansfield and the designators 1 and 2 are used to refer to two outlets of that lake at slightly different elevations

(Fig. 3B; Larsen, 1972, 1987). The transition from Glacial Lake Winooski to Glacial Lake Mansfield is evident

stratigraphically where parts of the lake floor that previously only received silt and clay size sediment abruptly began

receiving coarser sand.

As noted earlier, coarse sand and gravel was

deposited in deltas where streams entered Glacial Lake Winooski. One

delta occurs near the northwest corner of the quadrangle at Stowe Fork, another near the northeast corner of the

quadrangle where Moss Glen Brook enters the quadrangle, and a third in Stowe Hollow where Gold Brook formed a

delta. Streams flowing into Glacial Lake Mansfield also formed deltas that have been described earlier. However, the

majority of the sediments deposited in these lakes consist of medium to fine sand, silt, and clay. These fine

sediments were suspended in the high-velocity water emanating from beneath the retreating ice sheet and eventually

settled on the lake floor. These fine sediments were also sourced from streams eroding the recently deglaciated,

steep mountain slopes that were free of vegetation as well as the older Glacial Lake Winooski deltas.

Further retreat of the ice sheet WNW down the Winooski River valley allowed Glacial Lake Mansfield to drain and

merge with Glacial Lake Vermont, the large lower-elevation lake that occupied the Champlain valley (Van Hoesen et

al., 2016; Wright, 2018a). When this occurred, the low-relief, sediment filled lake bottom was

quickly exposed and

streams began to flow across them eroding new channels and depositing alluvium along their courses. Pre-glacial

stream channels were filled with glacial sediments. In most areas the modern streams re-excavated those old

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 18

channels, but in some areas modern streams eroded new channels. In the Stowe Quadrangle, the old Little River

channel remains buried beneath over 200 ft of sediments that currently extend south from Stowe High School along

Barrows Road. Formerly, the West Branch joined the Little River just north of the high school and joined rivers flowed

south from there to rejoin the modern channel where Barrows Road intersects Moscow Road. The “new” channel that

the Little River eroded crosses bedrock in Stowe Village. This bedrock is the source of the water fall that powered the

mills critical for Stowe’s early development. This bedrock has also prevented the Little River from eroding the glacial

lake sediments from the valleys upstream of the village. Instead, both the West Branch and Little Rivers have

meandered back and forth across the glacial lake sediments that filled the bottom of the glacial lakes that once

existed here. These wide, low-relief valleys with a thin cover of river sediments have historically provided the best

agricultural land in the area.

Groundwater Hydrology of the Stowe Quadrangle

Within the Stowe Quadrangle, people rely almost entirely on groundwater for their drinking water supply. Most

households utilize private water wells, but the villages of Stowe and Waterbury Center operate municipal water

systems that also are sourced from groundwater wells. Surficial aquifers are utilized by some residents (shallow dug

wells or deeper drilled wells in surficial materials), however drilled wells extending variable depths into bedrock are

used by most residents.

Several different types of bedrock underlie the town. While they differ in their mineralogy and history, they are all

metamorphic rocks and have no primary porosity, meaning there is no open space between the mineral grains in

these rocks to store water. Consequently, groundwater in these rocks is located in fractures and any drilled well in

bedrock gets its water from fractures intersected by that well. Generally, the volume of groundwater in fractured

bedrock aquifers depends on the density of fractures but is typically less than 1% of the rock volume. On the other

hand, most surficial materials have a lot of primary porosity, i.e. a lot of open space between individual sediment

grains, typically 25–45% of the volume of the sediments. The usefulness of water in those pore

spaces depends on

how easily water can move through these surficial materials, i.e. how permeable these sediments are. Generally,

groundwater moves very slowly through fine-grained materials and much more quickly through coarse-grained

materials.

The largest useful groundwater aquifers occurring in the quadrangle are found in the coarse-grained surficial

materials (sand and gravel) where those materials extend below the water table. The sand and gravel deposited in

deltas that were deposited in both Glacial Lakes Winooski and Mansfield are one of these aquifers. The second large

group of aquifers are the ice-contact sediments (sand and gravel) that lie buried beneath fine-grained glacial lake

sediments in many of the area’s valleys. These sediments, while buried and only discoverable via drilling or

geophysical work, are confined and consequently isolated from contamination from human and agricultural sources.

Water Table Contour Map with Flow Lines

A map contouring the elevation of the water table is included with this report. The data used to construct these

contours comes from the topographic map of the area and domestic water wells where the depth to the water table

was recorded. Specifically, groundwater

discharges to the surface in streams, ponds, lakes, and wetlands so these

are areas where the elevation of the ground surface and the elevation of the water table are equal. In areas between

these groundwater discharge points the water table is, by definition, at an elevation below the ground surface.

Streams are common in the upland areas implying that even in these areas the water table is relatively close to the

ground surface. For the groundwater wells, the water table elevation was calculated by taking the surface elevation of

the well (garnered from the LiDAR DEM) and subtracting the depth to the static water table. The calculated water

table elevation (in feet above sea level) is labeled adjacent to each well where these data are available.

Drainage basin outlines were downloaded from the VCGI database and denote both surface water and groundwater

divides. In this mountainous terrain water table contours were drawn at 500-foot intervals across the quadrangle.

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 19

The elevation of the water table varies seasonally. It’s generally highest in the early spring when groundwater is

recharged by melting snow and rain. It’s generally lowest at the end of the summer/early fall when the combination of

lower summer rainfall and very high evapotranspiration rates from plants limits recharge. Consequently, water table

contour lines on a map shift seasonally, moving up in the spring and down during the summer months. This is why

many streams at higher elevations flow in the spring, but go dry during the summer.

Groundwater flows down-gradient (downhill) perpendicular to groundwater contour lines. Interpretive groundwater

flow lines (arrows) are drawn on the map showing the approximate directions of groundwater flow across the area. In

general, most of these flow lines begin in the upland areas and end at streams where groundwater discharges to the

surface. The flow lines can be used to understand the pathways groundwater has taken to reach domestic water

wells. The flow lines can also be used to interpret the different types of bedrock and surficial material groundwater

has flowed through. These different types of rock and surficial material are the sources of all the naturally-occurring

dissolved ions in groundwater. For groundwater contaminated with human/domestic animal waste or other toxic

chemicals, the flow lines can be used to search areas up-gradient from the contaminated groundwater for potential

sources. Note however that the detailed groundwater flow paths needed to show point sources of groundwater

contamination cannot be deciphered from this map.

Recharge Potential to Surficial Aquifers Map

Groundwater recharge depends on (1) how easily rainfall and snow melt can infiltrate the ground surface, (2) the rate

at which water can move through the surficial material or rock it infiltrates into—its permeability, (3) the amount of

time that water is available to enter different groundwater systems. Infiltration is enhanced when the ground surface is

permeable and rainfall and snow melt can linger on the ground surface. Coarse-grained surficial materials on level

ground provide the best infiltration whereas steep bedrock surfaces provide the worst. Vegetation, burrowing animals,

and frost heaving usually enhances infiltration by increasing the permeability of soils and providing a myriad of small

depressions where surface water can linger and infiltrate. Fine-grained surficial materials usually have a low

permeability. Even if vegetation allows water to infiltrate, the rate it

can percolate (seep) into the material may be far

slower than the rate at which new water from rainfall or snow melt is available. In fine-grained materials with low

permeability, e.g. many tills and lacustrine sediments, a significant amount of water can move though these materials

if enough time is available.

The recharge potential map included with this report groups surficial materials into 1) those with high porosity and

high to moderately high permeabilities and 2) those with low permeabilities.

High Recharge Potential to Surficial and Bedrock Aquifers

Alluvium, Alluvial Fan sediments, Fluvial Terrace sediments, Deltaic sediments, and Wetlands are all materials that

readily absorb surface water. They all consist of coarse-grained surficial materials and lie in valleys where slopes are

gentle. Where these materials overlie moderate- to low-permeability materials, they have the capacity of soak up

surface water allowing it to slowly seep into these underlying surficial materials. With the exception of the wetlands

areas, these materials make excellent surficial aquifers where they extend below the water table. Wetlands uniquely

serve as good recharge sites because they occur in closed depressions where surface water collects. Even if the

surficial materials underlying wetland have a low permeability, they will have a near constant flow of well-filtered

surface water through them into the underlying groundwater system.

Low to Moderate Recharge Potential to Surficial and Bedrock Aquifers

Lacustrine very fine sand, fine sand, silt, most till and artificial fill (commonly covered with pavement) all have

moderate to low permeabilities. Till mantles most upland areas and usually directly overlies bedrock, so till itself is the

surficial aquifer that’s being recharged. As noted earlier, animals, vegetation, and frost heaving enhance near surface

infiltration and dug wells utilizing groundwater from till are common. Lacustrine fine sand, silt, and clay occurs in the

valley bottoms where slopes are gentle which enhances its ability to absorb water. Slow movement of water through

these materials can recharge coarse-grained surficial aquifer materials or bedrock, albeit slowly.

March 2020 Surficial Geology/Hydrology of the Stowe, Vermont 7.5-Minute Quadrangle 20

Bedrock Hydrologic Units Map

A “Bedrock Hydrologic Map” was generated using the bedrock map units utilized for the Bedrock Geologic Map of

Vermont (Ratcliffe et al., 2011). Rock units within the Stowe Quadrangle include several members of the Stowe

Formation (CZs, CZsg, CZswa, CZsws), the Jay Peak Formation (CZj), two members of the Hazens Notch Formation

(CZhn, CZhng), the Fayston Formation (CZf), and two members of the Ottauchechee Formation (Co, Coa). These

are all metamorphic rocks consisting of siltstone, shale, and basalt metamorphosed to produce a variety of

metamorphic rocks, e.g. phyllites, schists, greenstones, and amphibolites). Water wells from the database compiled

by the Vermont Agency of Natural resources are also shown on the map with the size of the well symbol correlative

with the well yield (GPM: Gallons Per Minute).

Water wells were grouped by rock type and statistics (maximum, minimum, median, mean, and total count) were

calculated for both well depth and well yield. These statistics include all wells occurring within each rock unit. Most of

these are drilled wells in bedrock, but some are deep wells that tap surficial aquifers. High-yield wells occur in all rock

units,

but some of these may be deep wells tapping surficial as opposed to bedrock aquifers. There is no significant

difference in well yield between the different rocks in the quadrangle reflecting the fact that the yield of any drilled well

in metamorphic rocks is most likely correlated to the density of fractures in the underlying bedrock (specifically the

number, width, length, and interconnectedness of those fractures) and not its mineralogy. Extensively fractured

bedrock is more susceptible to weathering and erosion than unfractured bedrock and frequently guides the location of

large- and small-scale valleys that are quite linear. These linear features can be mapped and used as a guide for

drilling high-yield bedrock water wells. A map of this type was not generated as part of this project.

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