B6: ICE RETREAT AND READVANCE ACROSS THE GREEN MOUNTAIN FOOTHILLS: BOLTON AND JERICHO, VERMONT
Stephen F. Wright
Department of Geology
University of Vermont
Burlington, Vermont 05405
swright@uvm.edu
George E. Springston
Department of Earth and Environmental Sciences
Norwich University
Northfield, Vermont 05663
John G. Van Hoesen
Department of Environmental Studies
Green Mountain College
Poultney, Vermont 05764
INTRODUCTION
This field trip visits several key sites in the towns of Bolton and Jericho Vermont that shed light on the glacial, ice-contact, and lacustrine environments that existed during a relatively short period of time during the retreat of the Laurentide ice sheet across this area. This area lies on the western slope of the Green Mountains, an area dominated by steep slopes and narrow stream valleys that are cut by the broad, low-gradient Winooski River valley (Fig. 1). Stop 1 is a section displaying evidence of at least one cycle of glacial retreat followed by readvance and eventual retreat. Stop 2 visits a working gravel pit where the internal structures of a large delta formed where glacial meltwater flowing south from Bolton Notch entered Glacial Lake Mansfield which occupied the Winooski River valley. Farther north in Bolton Notch Stop 3 visits a series of nested meltwater channels and other ice-contact landforms. The trip finishes at a large landslide section along the Lee River (Stop 4) which exposes the full transition from quiet water lake sediments through deltaic sediments where the Lee River delta prograded into the Coveville Stage of Glacial Lake Vermont. The location of all the field trip stops are shown in Figure 2 and more detailed maps accompany the road log. Many of the field stops described in this guide are located on private land. Permission from landowners, noted in the following guide, must be secured before visiting these sites.

B6: ICE RETREAT AND READVANCE ACROSS THE GREEN MOUNTAIN FOOTHILLS: BOLTON AND JERICHO, VERMONT

Meeting Point

Vermont Meeting Point

STOP 1. Preston Brook Landslide
Stop 1 is a large landslide along the east side of Preston Brook in Honey Hollow, a steep, narrow N–S valley typical of the Green Mountains (Fig. 5). There are two ways of accessing this field site. If the water level in Preston Brook is low enough, the bottom of the slide can be accessed by driving 0.5 mile up Honey Hollow road (see above road log) and parking at a small pull-off where a red iron gate blocks an overgrown logging road. The base of the slide is approximately 200 m SE of the iron gate. There is no trail. When Preston Brook is too high to cross, the top of the slide can be accessed by hiking ∼1 km up the Catamount Trail to where a gently sloping alluvial terrace separates the trail from the top of the landslide.
Wright first observed this section in 2008 and as of this writing the exposures are still quite good. Please anticipate that they may not remain so in the future. Continuing slope movements change which parts of the section are visible at different times. Parts of the section are very steep and quite wet. Caution and sturdy/waterproof footwear are recommended.
The landslide exposes an approximately 30 m high section containing lacustrine sediments at three stratigraphic levels separated by both diamict and coarser-grained ice-proximal sediments (Fig. 6). The description that follows divides the section into “Lower,””Middle,” and “Upper” parts which are separated by covered intervals. Lacustrine sediments exposed in the “Lower” and “Middle” parts of the section are deformed. One objective of this stop is to ascertain whether structures in these deformed sediments formed by (1) a readvance of the ice sheet or (2) whether they formed during one or more landslides (debris flows) in this narrow, steep-sided mountain valley.
Lower Section
The lowest part of the section (stream-level) consists of fine-grained lacustrine sediments that are folded, faulted, and over-consolidated (Fig. 6). These deformed sediments are overlain by a boulder/cobble/pebble-rich diamict(?). This in turn is overlain by a boulder/cobble gravel that generally fines upwards into a sequence of sediments consisting largely of silt and very fine sand, with irregular lenses of coarser sand, pebbles, and cobbles (Fig. 6). Bedding is sometimes coherent, but also occurs as a slumgullion of lenses and irregular blebs or isolated fragments of silt/clay or rocks surrounded by finer grained sediments. The finer sediments are very compact and appear to be over-consolidated. Near the top of the lower section an approximately 10 cm thick bed of laminated silt and very fine sand has been both folded and apparently injected (dike-like) into the overlying sediments (Fig. 7). To the right (south) of the fold hinge bedding is continuous at least a short ways down section. To the left (north) of the fold hinge bedding has been pealed/delaminated away from the underlying sediment and highly deformed sediment occupies the space above the now overturned limb of the fold. Figure 6: Description and interpretation of the Preston Brook landslide section that extends upwards from the brook to an abandoned fluvial terrace (see Fig. 3 for location). The section exposes three separate units of lacustrine sediments separated by both diamict and glaciofluvial sediments. The lower two units are deformed as a consequence of emplacement of the diamict as till during a glacial readvance or as till remobilized during an underwater landslide. The uppermost part of the section consists of alluvium deposited when Preston Brook flowed across the sediment-filled valley into Glacial Lake Vermont.
Middle Section
The middle part of the section is largely covered and water-saturated, but small exposures consist of silt and clay layers (varves) that have been deformed into meter-scale folds (Fig. 6). The present limited exposure makes it impossible to know whether (1) the entire section is deformed or (2) whether the deformation is intraformational (i.e. the folds occur only within one stratigraphic horizon and are overlain by undeformed sediments).
Upper Section
The upper part of the section consists of diamict (till?/landslide debris?) overlain by rhythmically bedded (varved) very fine sand, silt, and clay (Fig. 6). While some layers contain intraformational slumps, particularly at the base, this section is undeformed. The lacustrine sediments are overlain by a thin veneer of alluvium which extends across a gently north-sloping terrace (between the landslide headwall and the Catamount trail) and marks the top of the section. An old alluvium-filled stream channel cuts across the uppermost section of lacustrine sediments (Fig. 6).
Interpretation
The Preston Brook valley (Honey Hollow), similar to other valleys in the Winooski River basin, was partially flooded by Glacial Lakes Winooski, Mansfield, and Vermont (upper Coveville stage) as the ice sheet retreated NNW (down) the Winooski River valley (Fig. 4). The top of the landslide scarp is at an elevation of ∼600 feet (∼185 m) which is at or below the elevation of all these lakes (Fig. 6). It is unclear which of the lacustrine sediment units exposed in this section were deposited in which of these lakes or within a lake that may have preceded the formation of Glacial Lake Winooski. In the interpretation that follows, we have suggested a sequence of events that is consistent with the sediments exposed in this section and our understanding of the glacial history in the Winooski River valley, but other interpretations are also possible.
The fine-grained lacustrine sediments at the bottom of the section indicate that at the time these sediments were deposited, the ice sheet had retreated down the Winooski River valley west of the mouth of Preston Brook in order to flood the valley with lake water. We suggest that this lake formed in the Winooski River valley before the Middlesex Readvance that Larsen (1999) referred to as Glacial Lake Merwin. The tilted, faulted, folded, and over-consolidated nature of these sediments, in conjunction with the overlying diamict, suggest that they were overridden during a readvance of the ice sheet into the Preston Brook valley, the Middlesex Readvance. This readvance occurred during an approximately 100-year long cold period between 13,900 and 14,000 yr b2k and is coeval with the Littleton Readvance in northwestern New Hampshire (Ridge et al., 2012, Thompson, 1999). During this readvance the ice sheet reclaimed most of the Winooski River drainage basin as evidenced by sections of deformed sediments in many of the river’s tributary valleys (Larsen 1999, 2001; Larsen et al. 2003; Wright 1999, 2002, 2015).
Glacial Lake Winooski began to form at the end of the readvance when the ice once again began retreating WNW down the river valley. The coarse boulder/cobble gravels that overlie the lower diamict suggest they were deposited near the mouth of an esker tunnel as the ice sheet retreated across this area. The fining up sequence of fluvial sediments that overlie the coarse gravels were most likely deposited in a subaqueous fan that became progressively finer grained as the ice sheet retreated west of the Preston Brook valley. Continued retreat of the ice established a quiet-water arm of Glacial Lake Winooski in the valley when the varved silt/clay sediments in the middle section were deposited. However, by the time the ice retreated far enough down the Winooski River valley to expose the Preston Brook valley only 2 km of additional ice retreat allowed Glacial Lake Winooski to catastrophically drain through the Huntington River and Hollow Brook valleys across two different thresholds to form Glacial Lake Mansfield (Fig. 4). Elsewhere in the Winooski River valley this initial lake elevation drop of ∼80 m is stratigraphically instantaneous and clearly recorded by an abrupt increase in grain size (Larsen et al., 2003). Very little of the middle part of the section is exposed, but we suspect that these fine grained, ice-distal, lacustrine sediments were deposited in both Glacial Lakes Winooski and Mansfield and, if better exposed, would show the transition between the two lakes.
The large folded layers of silt/clay visible in the exposed part of the Middle Section may have formed by slumping or they may have formed from shear stresses imparted when the upper diamict was emplaced. One interpretation is that the diamict formed from a debris flow (consisting of remobilized till mixed with lacustrine sediments) originating on the steep valley side that flowed into the lake-flooded valley. After entering the lake the debris flow triggered further slumping/deformation in the already-deposited sediments now exposed in the lower and middle parts of the section (Fig. 7). This slumping may have been triggered by the catastrophic drainage event associated with the transition from Glacial Lake Winooski to Glacial Lake Mansfield.
If the upper diamict is interpreted as a debris flow deposit, then the lacustrine sediments deposited on top of the diamict are most likely Glacial Lake Mansfield sediments as they were deposited in relatively quiet water and the elevation of Glacial Lake Vermont (Coveville Stage) in the valley is only slightly higher than the elevation of these sediments (i.e. when Glacial Lake Vermont occupied the valley this area would have been close to its shoreline).
Another interpretation is that the diamict is a readvance till and deformation in the underlying section was produced by movement of the overlying ice sheet. If this diamict was produced during the Middlesex Readvance, then all the underlying sediments are older than Glacial Lake Winooski and the thin section of lacustrine sediments lying above the diamict were deposited in Glacial Lake Winooski. It is also possible that this diamict records a more recent readvance of the ice sheet, although there isn’t any indication in the ice core records of another cold period capable of producing a readvance during the waning stage of Glacial Lake Winooski (∼13,700 yr b2k). Unlike the lower diamict, the upper diamict is overlain by sediments deposited in quiet water. If this upper diamict is a till deposited during a readvance, subglacial conduits did not funnel high-energy sediments in the valley as the ice was retreating.
The uppermost lacustrine sediments are cut by a large channel filled with a poorly sorted pebble/cobble/boulder gravel and are overlain by a thin layer of similar sediments interpreted to be historical Preston Brook alluvium. The abandoned stream terrace at the top of the section grades to the elevation of the Coveville stage of Glacial Lake Vermont (Fig. 6). This indicates that the valley was locally filled with at least 100 feet (30 m) of sediment when Preston Brook began incising its present channel when the elevation of Glacial Lake Vermont dropped from the Coveville to Fort Ann stage.

STOP 2. Bolton Notch Delta
The active gravel pit on the west side of the road is excavating deltaic sediments deposited by a north-flowing stream originating in Bolton Notch. This is a large delta and many of the foreset and topset beds are composed of very coarse gravels with some very large (m-scale) lag(?) boulders (Fig. 8). The size of the drainage basin is small which suggests that the high-energy stream that built the delta was fed directly by melting ice retreating up the notch and/or by drainage out a glacial lake that formed on the north side of the notch (Fig. 9). No deltas occur (or are not preserved) in other stream valleys farther east suggesting that the Bolton Notch delta formed quickly during a time span too short for significant non-glacial deltas to form in adjacent stream valleys.
Well-developed foreset and topset beds are usually exposed in the pit. These are overlain by a thin (∼2 m) sheet of medium to fine sand. Duck Brook has eroded a deep channel through this delta isolating an erosional remnant of this deltaic terrace on the west side of the brook. The elevation of the topset/foreset contact, measured using a Trimble GPS, is ∼233 m (± 1 m). With ∼4 m of isostatic uplift (based on an almost due north uplift slope of 0.7 m/km (Rayburn, 2004), this corresponds with the higher 229 m Gillette Pond outlet of Glacial Lake Mansfield (Larsen, 1987, Fig. 7).
The geometry of the ice margin at this time suggests that the Gillette Pond outlet was tenuous at best (Fig. 9).The medium to fine sand overlying the coarse topset beds of the delta was most likely deposited when the lake deepened for a short period of time, drowning the delta (i.e. the delta moved farther up the Bolton Notch valley and only fine-grained bottomset sediments here). A minor readvance of the ice sheet could easily have closed the Gillette Pond outlet allowing the lake elevation to rise for a limited period of time (Fig. 9). Similarly, a relatively small 1–2 km retreat of the ice margin uncovered a lower elevation outlet (Hollow Brook) which allowed the lake elevation to drop ∼25 m.

STOP 3. Bolton Notch
Bolton Notch is a north-south U-shaped valley bounded by near vertical bedrock cliffs on either side of a valley floor filled with an unknown thickness of talus, lacustrine, and fluvial sediments. A poorly drained part of the valley floor marks the drainage divide at an elevation of 1180–1200 ft (∼365 m) asl (Fig. 10). The objective of this stop is to observe and offer interpretations of (1) a series of terraces and channels that likely formed during ice retreat, (2) a group of kettles occurring immediately south of the gravel pit, and (3) ice-contact and lacustrine sediments exposed in the gravel pit. All of these features occur at or immediately north of the drainage divide. A detailed map of many of the small-scale landforms occurring near the drainage divide of the notch was made at an original scale of 1:4,000 using a hand-held GPS device (Fig. 11).
Striation Data
Striation measurements in the area surrounding the notch indicate a very consistent NW to SE direction of ice flow (Figs. 3, 10). Few measurements have been made on the steep valley sides; however some striations on the lower slopes show deflection to a NNW to SSE direction suggesting that ice flow rotated parallel to the valley as the ice sheet thinned, although the length of time the ice flowed in this direction was probably relatively short. Therefore, despite the distinctive “U” shape of the valley, this form may be more strongly linked to differential weathering and erosion of a relatively weak lithology underlying the valley rather than erosion by moving ice.
Terraces
A nested group of terraces occur along the eastern side of Bolton Notch, near the drainage divide. They are all aligned NE–SW, oblique to the N–S orientation of the valley, and all slope gently to the southwest (Fig. 11). The southernmost terraces are the highest and they progressively step down to lower elevations farther north. The lower-elevation terraces cross-cut adjacent higher-elevation terraces. The elevation difference between adjacent terraces typically ranges between 2–5 m. The most northerly of the terraces leads into a distinctive channel (now flooded) that occupies most of the area at the drainage divide. Farther south this pond drains and forms the modern stream channel that cross-cuts all of the higher terraces (Fig. 11). Most of these terraces are floored with till, but some also contain poorly rounded cobbles and boulders, particularly notable where these materials have been incorporated into stone walls.
These nested terraces are interpreted to be abandoned glacial meltwater channels that formed from water discharging from a tunnel beneath the glacier. This water flowed between the retreating ice front and the steep valley side or older, higher terraces. From south to north, these different terraces, at successively lower elevations, mark the position of the retreating glacier, perhaps at yearly intervals (Fig. 11). It was this glacial meltwater, flowing south through the notch and towards the Winooski River valley, that supplied most of the sediment that accumulated in the Bolton delta (see Stop 2).
Kettles
Several kettles occur in the bottom of the valley south of the gravel pit and two of the larger ones are shown on the detailed map (Fig. 11). Several of the abandoned meltwater channels abruptly drop into the pond suggesting that the pond too is a kettle or network of kettles and streams emanating from the glacier flowed directly across stagnant ice. The implication is that as the retreating ice thinned across the drainage divide, it thinned enough so it could no longer flow and became stagnant and disconnected from the thicker, still active ice front north of the divide.
Gravel Pit Geology
An old gravel pit lies along the north side of the road, at the northern end of Bolton Notch (Figs. 10, 11). The pit exposes sediments deposited in both an esker and a small glacial lake that formed as soon as the ice front retreated north of the drainage divide. The trace of the esker is shown on Figure 11. While many of the pit faces are slumped, an amazing vertical walled “canyon” has formed between two levels of the pit and offers excellent exposures of sedimentary structures (Fig. 12). Material eroded from this canyon has formed a large alluvial fan immediately below the northern end of the canyon.
Much of the esker has been quarried away, but many large rounded boulders occur on the floor of the pit at its southern end and very coarse sediment with additional boulders occurs along the pit’s slumped face immediately below and northeast of the road. Many of the boulders deposited in the esker tunnel are high-grade metamorphic rocks (Grenville-age gneisses) that are distinctly erratic to northwestern Vermont and were transported here from the northern Adirondack Mountains or the Laurentian Mountains north of Montréal.
The bulk of the material currently exposed in the pit consists of medium to very fine sand and silt with occasional, relatively thin beds of coarse sand and pebble gravel. Dropstones are common. Cross-bedding and climbing ripples, frequently in beds >1 m in thickness, occur in many beds and all indicate that currents were dominantly flowing to the south, towards the notch. Structureless beds of medium to fine sand imply that these beds are slumped. Coherent blocks of bedded sediment have also slid off the steep side of the esker into deeper water (Fig. 13). There is a general fining-up pattern where the thickest beds of coarse sediment occur in the deepest parts of the pit and the finest interlayered fine sand/silt horizons occur at the uppermost part of the pit. Soft sediment deformation by slumping and/or differential loading is extensive in some beds.
The fine lacustrine sediments exposed here indicate that a small glacial lake existed here once the ice sheet retreated north of the drainage divide (Figs. 10, 11). The lowest elevation channel, leading to and through the pond, marks the final outlet to this lake at an elevation of ∼1,190 ft/363 m (Fig 11). This lake was likely small and short-lived as less than 1 km of further ice retreat to the northwest would have uncovered a much lower outlet to the Winooski River valley via the Indian Brook drainage (outlet elevation 920 ft/280 m).
A ground penetrating radar (GPR) survey shows that in the central part of the pit an esker is buried by a sequence of lacustrine sediments that outcrop in most of the pit (Fig. 14). Another GPR survey line emphasizes the continuity of these beds (Fig. 15). The 100 GHz antenna shows coherent reflectors to a “depth” of 300 ns (Fig. 14) whereas penetration with the 200 GHz antenna does not extend much below 150 ns (Fig. 15). However, the 200 GHz antenna shows sedimentary structures in much more detail.
The large volumes of sand, frequently deposited as meter-thick beds of climbing ripples from south-flowing currents, implies that sediments in this glacial lake originated from a subglacial drainage system, i.e. meltwater and sediments discharging through a tunnel, which formed a subaqueous fan that fined upwards as the ice front receded to the north. Given the small size of the lake basin and the high sedimentation rates at the mouth of subglacial tunnel, the lake probably rapidly filled with sediment. At the northern end of the canyon a large >15 m wide) channel filled with coarse sand and pebble/cobble gravel cuts across the finer lacustrine sand. This is interpreted to be a stream channel extending from the receding glacier to the drainage divide that was eroded through the recently deposited lacustrine sediment that largely filled the lake basin.

STOP 4. Lee River Delta (2 hours)
A large landslide has been recurrently active along an outside meander bend of the Lee River for at least the last 15 years revealing an almost 100 ft (30 m) high section (Fig. 16). From year to year exposures vary, but key parts of the section can be excavated where slumped. The section offers the opportunity to see the almost complete transition between relatively deep water lacustrine sediments upwards through a classic Gilbert-style delta which was deposited in the higher, Coveville stage, of Glacial Lake Vermont. This site lies on private property. Please contact the McLoughlin family (Plains Road, Jericho) for permission to visit the site.
The lowest part of the section is frequently slumped, but consists of rhythmically bedded silt and clay (varves) with occasional dropstones. Bedding is approximately horizontal. Farther up the section the summer silt layers frequently contain beds of medium to fine sand, some of which contain well-preserved asymmetric ripples. Ripples and cross-beds here and farther up the section consistently indicate current flow was to the northwest at the time these sediments were deposited, parallel to the modern course of the Lee River.
The proportion of sand in the section rapidly increases upsection. Winter clay layers are usually present and indicate that the annual accumulation of sand often exceeded 1 m/year. Asymmetric ripples, climbing ripples, and graded beds are common in these still horizontally bedded sediments (Fig. 17). Soft sediment deformation resulting from differential loading has produced folds and flame structures in some layers. Calcite-cemented concretions are also common. These are the bottomset beds of a delta encroaching from the east, fed by the Lee River.
Higher in the section the first beds containing coarse sand and pebble gravel appear bedding is inclined to the west northwest. The average grain size continues to increase in these foreset beds. The very top of the section consists of horizontal beds of coarse pebble/cobble gravel, the topset beds of the delta. Poor exposure usually makes it difficult to ascertain whether the transition between the foreset and topset beds is smooth or truncated. The topset/foreset contact lies ∼2 m below the terrace at the top of the landslide. The elevation of that terrace has not been surveyed, but lies between 620–640 ft (189–195 m) asl based on the topographic map and pins the elevation of the Coveville Stage of Glacial Lake Vermont in this part of Vermont (Fig. 18).