Although Ice Age glaciers covered the Champlain Valley over 10,000 years ago and helped carve out Lake Champlain, their actions were superimposed on a landscape already prepared to host a lake.
Two factors made what is now the Champlain Valley better suited to host a large lake than any of the surrounding area between Lake Ontario and the Connecticut River. First, a valley probably already existed here. Second, the bedrock was softer and easier to erode than in the surrounding mountains.
We begin the story of the pre-existing valley 160 million years ago. At this time, all the continental plates were mashed together in the supercontinent known as Pangea with what was to become the Champlain Valley near the center. The Green Mountains already towered over the area. The Adirondacks we know had not yet arisen.
The ancient supercontinent of Pangea was beginning to break apart. As the continental plates separated, oceanic crust formed between them and the Atlantic Ocean grew. As the plates pulled apart, the crust stretched like pizza dough. In some places, lava squirted through rips in the crust forming volcanoes. In other areas rock layers cracked and shifted forming faults. Many faults lay along what is now a north south orientation, parallel to the crust's movement.
Between two of these parallel faults, large blocks shifted downward forming a graben (German for ditch). Imagine holding a deck of cards perpendicular between your hands. By squeezing the cards you can keep the deck whole, but if you relax your grip the center portion slips downward. The breaks between the cards represent faults. The cards that fall are the graben. One example of the graben-forming fault can be seen at the Palisades, the cliffs opposite the mouth of the Otter Creek. In a span of a mere mile or two, the rocks change from flat limestones on Vermont to much older metamorphosed rocks of the Adirondacks. The limestones in the valley clearly dropped many thousands of feet relative to the adjacent Adirondack rocks.
Even millions of years before the glaciers arrived, the graben had likely widened and deepened. The surrounding mountains would have prevented water from escaping in other directions. The ancient rivers that drained these mountains left few traces. However, water flowing off them would have sought planes of weakness in the rocks of the graben, eroding the ditch over time.
Such erosion would have been facilitated by the second factor in our tale, the relative softness of the underlying limestone and dolomite bedrock. Limestone and dolomite are both calcium carbonate based rocks. Calcium carbonate dissolves in weak acids, and rainwater is naturally acidic because of the presence of carbon dioxide in the atmosphere.
Over time these factors can tremendously weaken rock. For example, many of the world's most impressive natural cave formations, like Mammoth Cave in Kentucky, occur in limestone. Meanwhile the bedrock of the surrounding mountain is quartzite and granite, rocks much harder and more resistant.
The valley bedrock formed, long before the graben dropped, long before Pangea had formed, somewhere between 600 and 450 million years ago. At that time what is now the eastern United States was covered by a warm shallow ocean. Warm because the entire area was located closer to the equator. Shallow because the ocean water sat over the edge of the continental crust, like the Mediterranean Sea or Bahama shelf does today, rather than over deep crust like that of continental mountain belts.
Meanwhile, corals and other sea creatures inhabited this ocean with shells of calcium carbonate. Over millions of years their shells fell to the sea floor when the organisms died, mixed with sediments washing in from the surrounding land, and with time and pressure formed today's bedrock. Meanwhile, the entire region drifted northward upon shifting tectonic plates to its present location. Remnants of these early coral reefs with fossils intact can still be found throughout the Champlain Valley, with some of the best examples preserved on Grand Isle and Isle la Motte in the middle of Lake Champlain and at Crown Point N.Y.
So when the glaciers arrived the stage had already been set. The graben had produced a valley through which they could flow. Soft sedimentary rocks comprised the bedrock of that valley. Meanwhile, years of rainwater and running water had weakened the bedrock. As the ice sheets oozed through, pushing their layer of rocks beneath them like sandpaper, it was easy enough to deepen the Champlain Valley before filling it with melt water when they finally retreated.
Special thanks to the Lake Champlain Committee for the use of this news article. LCC is a 2,500-member citizens' conservation organization that has been working in New York, Vermont and Quebec for over 40 years to protect the Lake Champlain environment. You can get involved with lake protection by joining LCC. Join online using a web site secure form at: www.lakechamplaincommittee.org) or call (802) 658-1414 for details.