The topography visible today throughout the Basin is the remnant of mountain building and erosion from glaciers that long ago passed through the valley and scoured the surface of the mountains. Most of the Basin can be divided into five distinct physiographic regions: the Champlain Lowlands or Champlain Valley, the Green Mountains, the Adirondacks, the Taconic Mountains, and the Valley of Vermont, as shown on the Physiographic Regions Map [938 KB]. The Vermont Piedmont and Hudson Valley regions make up small portions of the Basin.

The Green Mountains, Taconic Mountains and Adirondack Mountains contain the highest peaks surrounding Lake Champlain and form the headwaters of many rivers that flow to Lake Champlain. Most of the water that falls on these mountains as rain or snow will eventually end up in the Lake. The Valley of Vermont is a small section of rich agricultural lands between the Green Mountains and the Taconic Mountains that also supplies water to Lake Champlain.

The Iapetus Ocean, an ocean over 500 million years older than the present day Atlantic, once flowed in what is now the Champlain Basin. It was a warm, shallow sea. Evidence for this ancient ocean includes sedimentary rocks such as sandstone and limestone, and marine fossils, such as the Chazy Reef in Isle La Motte, Vermont, known as the world’s oldest reef.

Over 400 million years ago the Iapetus Ocean closed, and the sedimentary rocks of the shoreline and continental shelf were folded and faulted to form the Green Mountains, part of the Appalachian Mountain chain. The great stresses of mountain building altered the older sedimentary rocks by heat and pressure into metamorphic rocks such as schist, marble and slate. The huge compressive stresses of mountain building also caused portions of the earth’s crust to break and move as large fault blocks. The sedimentary rocks of western Vermont and the Taconic Mountains are examples of these large fault blocks, where younger rocks have been pushed up and over metamorphosed continental shelf rocks beneath.

The Adirondack Mountains are sometimes called new mountains from old rocks. The Adirondacks are a unique dome formation, which started rising about 5 million years ago. As the dome rose over time, newer rocks were eroded, exposing the present day rock, which was formed more than one billion years ago. This rock is primarly anorthorsite, which is similar to rocks found on the Moon. Recent surveys suggest that the Adirondacks may still be rising at a rate of 2-3 mm annually, which is 30 times faster than the rate of erosion!

The Basin's last significant geologic event occurred relatively recently in geologic time with the beginning of the Great Ice Age. The Earth cooled dramatically during the Ice Age and glaciers advanced throughout the region. The ice descended into the Basin about 20,000 years ago, covering all of the Basin and most of New England and New York State with a sheet of ice about one mile thick!

The movement of the ice caused intense erosion and weathering of the mountains, which is shown by their rounded shape today. Evidence of glaciation can be seen throughout the Lake Champlain Basin, from rounded glacial valleys to eskers (rocky ridges resulting from stream deposits).

When the earth began to warm some 12,500 years ago, Lake Vermont formed from melted glacial waters. The extent of this lake is shown on the Lake Vermont Map [166 KB]. During this time period, both present day Plattsburgh, NY and Burlington, VT were under water and Mount Philo was an island in this long-ago lake. A record of layered lake bottom clays is further evidence of Lake Vermont.

Approximately 10,000 years ago, the glacier's retreat allowed marine waters from the St. Lawrence estuary to flood the Basin, forming the Champlain Sea, an arm of the Atlantic Ocean. The extent of this salty sea is shown on the Champlain Sea Map [182 KB]. Many marine animals, including Beluga whales, Atlantic cod, seals, and blue mussels lived in the Champlain Sea. In 1849, railroad workers found a Beluga whale skeleton in Charlotte, VT, which is now on display at the University of Vermont. Many other fossils of the Champlain Sea time period have been found in Canada.

Removal of the glacial ice, which was extremely heavy, allowed the earth’s surface to rebound. This rebound cut off the supply of salt water. The Champlain Sea gradually changed back into freshwater from rainfall, creating present day freshwater Lake Champlain, which has existed for about 9,000 years. The Basin is rimmed with sand and gravel deposits which record the shorelines and deltas of both Lake Vermont and the Champlain Sea.

• The Charlotte Whale Website
• Perkins Museum of Geology at UVM
• Vermont Geology
• Adirondack Park Geology (APA)
• Champlain Sea Fossils