Lake Champlain started forming 600 million years ago. A trough was formed when the tectonic plates that had collided one billion years ago (causing the uplift that formed the Adirondack Mountains) pulled apart (Harris, 1990).
The massive Wisconsin glacier covered all of New England 15,000 years ago and more than a mile of ice capped portions of northwestern Vermont. These millions upon millions of tons of ice depressed the earth's crust, bringing it below sea level. Some 12,500 years ago the glacier retreated north of the St. Lawrence lowland. Glacial striations and glacial till are still visible where the Pleistocene glaciers were funneled by the lake trough and then retreated. Large rocks remaining from the glacial till are called erratics and can be found dotted along the southern shoreline.
As the glacier retreated, saltwater flowed in from the Atlantic Ocean to fill the depression that is now the St. Lawrence Seaway. The Champlain Sea was created. An arm of the sea extended into what is now known as the Champlain Valley where it remained for approximately the next 2,300 years. A skeleton of a Beluga (also known as White) whale (Delphinapterus leucas), believed to have lived 10,000 to 12,500 years ago, was found a mile inland from the Point and serves as proof (Howe, 1997). Many of Vermont’s 77 native fishes arrived at this time (Langdon, Ferguson & Cox, 2006).
Released from the great weight of the ice, the ground slowly rebounded, and the ocean water began to flow north. It was replaced by fresh water melting into the valley and eventually the Champlain Sea disappeared. Lake Champlain has existed in its present form for about 9,000 years (Howe, 1997). It exits across the extensive deposits of glacial sand and silt that once covered the bottom of the Champlain Sea through the Richelieu River flowing north into the St. Lawrence Seaway to the Atlantic Ocean. The deepest part of the trough lies between Thompson’s Point and Split Rock, where lake bottom is 400 feet below the lake surface and bedrock is 1000 feet below.
The rock type is sedimentary dolostone, a grey-black limestone-like stone which is high in calcium and magnesium-carbonate. Thompson’s Point is the type location of the youngest formation, Cutting dolostone, which is the only dolostone horizon to hold fossils. The fossils are a small snail, Ophileta, of which only casts remain. (For further description of dolostone see Harris, 1990.) Flint lies between the layers within the dolomite.
Today, Thompson’s Point is a peninsula of land on the Vermont side, about 1.5 miles long, easily recognized from the Lake, air, and land . The Point is clearly defined by its bedrock base - 40 foot bluffs on the northwest shore that gently descend to a ledge on the south shore (Harris, 1990). The tip of the Point marks the deepest part of the Lake (130 meters). On either side of the Point are wide bays. Converse Bay to the north is moderately deep (4 to 100 meters), with a stone and pebble base. Two small islands are in it. No significant streams or rivers flow into it. The southern bay called Town Farm Bay is more shallow (1 to 22 meters) with more inlets and small bays. The base of the bay is filled with a marsh into which Thorpe and Kimball Brooks enter. The far side of the bay is another larger marsh through which the waters of a river (Otter Creek) and Lewis and Little Otter Creeks enter the Lake. The area is ideal for different life stages of many kinds of fish because of the confluence of different habitats—rivers, large and small bays, rocks and shoals, and wetlands—within a shallow bay next to the deep cold waters of the trough. Town Farm Bay has been considered one of the two most important fishing grounds in Lake Champlain since pre-historic times.