Summary of the Glacial Geology of Valley

Laurentide Ice Sheet

The Winooski River’s

flow has been temporarily halted

numerous times when massive ice

sheets extended down from

Canada, blanketing .

During the most recent ice age,

about 24,000 years ago, the 2.5

Figure 1. View of Camel’s Hump from the East. Ice flowed km (over 8,000 feet) Laurentide from right to left in this photo, plucking rock from the south side of the summit and creating a cliff. Source: http://www.nature.nps.gov/nnl/Registry/USA_Map/States/Vermont/NNL/CH/ ice sheet covered . As images/Camel's%20Hump-VT.jpg

this sheet flowed to the southeast, it drastically sculpted the entire landscape. Even the distinctive shape of Camel’s Hump is a glacial feature called a Roche Moutonnee, caused by ice moving over the summit

(figure 1).

Around 20,000 years ago, the climate warmed rapidly and the ice sheet began to thin, allowing higher elevations to become ice-free (Wright). As the ice further melted, a large glacier remained in the Champlain Valley and a tongue of ice extended up the

Winooski Valley. With rivers flowing in the very upper reaches of the Winooski

Drainage basin, water rapidly began to back up behind the ice-clogged valley.

Glacial Lake Winooski, Mansfield and Vermont

Figure 2. Approximate shoreline of Glacial Lake Winooski. Solid triangles denote the location of known delta built into lake Winooski. Adapted from Larsen (1999).

The glacier receded from the Upper Winooski Valley 14,000 years ago, allowing the water in the valleys to converge and form the extensive Lake Winooski (Springston,

2009). The outlet for this extensive lake was at the top of the Steven’s Branch of the

Winooski River, two miles south of Williamstown, VT (figure 2). This drainage divide, at 915 feet, is the lowest in the upper Winooski Valley and represents the height of the lake surface at the time. It was at this location that water spilled out of the Winooski

Valley and into the Connecticut River watershed.

Glacial Lake Winooski existed until the ice margin in the Winooski valley retreated past Jonesville, VT. At this point, water was able to flow into the Huntington

River Valley and down into the Hinesberg area and the Champlain Valley. Lake Mansfield formed as water rapidly equilibrated to around 650 feet, almost 300 feet lower than its previous level. Water from Lake Mansfield drained into the massive Lake

Vermont, another glacial lake caused by an extensive glacier in the Champlain Valley blocking the flow of water north towards . Glacial Lake Mansfield existed until the glacial margin moved northwest of Richmond, allowing the upper Winooski River to flow freely, while the lower Winooski Valley was inundated with water from Glacial

Lake Vermont.

Champlain Sea

Lake Vermont catastrophically drained 12,000 years ago and water from the St.

Lawrence seaway rapidly refilled the valley with salt water to an elevation of around 350 feet, forming the Champlain Sea (Wright). At the time, the land was considerably lower than it is now, as the region was depressed by the overlying weight of the ice sheet. The area around the Burlington Airport was at the edge of the sea, where the mouth of the

Winooski deposited large volumes of sediment, as evidence by the delta deposit that created the large terrace. With the weight of the overlying glacier gone, the land slowly rebounded, and the Champlain Sea slowly drained, allowing freshwater to fill the

Champlain Valley over a period of 2,000 years. The lower Winooski River and Lake

Champlain as we know it today has been in existence for only the past 10,000 years

(Wright).

Glacial Sediment

The last glacial period drastically shaped the landscape by eroding mountains and carving valleys. Much of this material was left behind as the glacier receded to the north. Tons of sediment, deposited by both the receding glacier and glacial lakes, filled all parts of the Winooski Valley. Unsorted sediments of varying size, called till, was deposited directly by the glacier. In valleys that were covered by glacial lakes, sand, silt and clay were deposited in layers over 100 feet thick in places.

The Winooski River has quickly transported parts of the glacial sediment out of the watershed. Over the years, the Winooski and its tributaries have been able to meander through these deposits, frequently changing channel locations throughout the valley. To this day, the Winooski continues to carry these glacial sediments downstream.

Rivers are conveyor belts for sediment, and the Winooski has been very active in this regard. In areas where the river has not flowed, diverse glacial deposits can be found that tell a complex story of ice, water and rock.

Sources:

Larsen, F.D., Glacial History of the Montpelier, VT, 7.5 Minute Quadrangle, in Wright, S.F. ed, New England Intercollegiate Geologic Conference Guidebook, Number 91, p 286-300.

Springston, G., Time Travel on the Winooski, Sojourn: Winooski River Guide 2009. Friends of the Winooski River, 2009, p. 7-9. http://www.winooskiriver.org/assets/2009%20Winnoski%20River%20book%20final.pdf.

Wright, S.F., Glacial Geology of the Burlignton and Colchester 7.5’ Quandrangles, Northern Vermont, http://www.anr.state.vt.us/DEC/geo/pdfdocs/GlacGeoBurlwright.pdf.