Over 450 million years ago, a shallow tropical sea covered the present-day Lake Champlain Valley. The sandy beach solidified into sandstone. In some places, the sandstone preserves the ripples formed by shoreline waves; in others, the ripples record the action of a stream, with the steep sides indicating the direction of flow. Marine creatures that settled to the ocean floor calcified into the other local bedrock, layered limestone named Chazy, or Beekmantown or Crown Point for places where geologists first identified the particular formations known as type sections. Curious visitors can find rock containing beautifully-preserved fossils of giant snails, trilobites and other sea life anywhere that weather, waves or waterfalls have broken apart this evenly-stratified gray/blue rock. Fossil hunters must not remove rocks from beaches on state land, such as those at Crab Island or Point au Roche that are in the public trust.
Continental collisions and mountain building formed much of the present-day physiography of the valley. About five million years ago, the Adirondack Mountains began to rise in a dome formation, known as a massif --- an uplifted area of ancient metamorphic rock formed at high temperature and high pressure 19 to 25 miles underground. Called the Adirondack Shield, it consists mostly of anorthosite, quartzites, and marble, and forms part of the Grenville orogen (mountain belt), extending underground for 3,100 miles from the eastern United States to Mexico.