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.