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.