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Geologic history of the Champlain Valley
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This page presents a brief look at the geologic development of the Champlain Valley, starting around 500 million years ago and running through the present day. The diagrams below represent simplified cross-sections of the region from west to east, running roughly from the central Adirondacks through the central Green Mountains. In places, maps are used to model what how the world's land masses were likely arranged at that time. Please note that the diagrams and descriptions are greatly simplified from the incredibly complex geology of the real world, and are intended only as a general overview of important events and concepts in the development of the Champlain Valley. All times are broad estimates, not exact dates. (Cross-sections drawn by Eric Butler)
600-500 million years ago
The region that is now Vermont would have been unrecognizable. The land mass that will one day form the core of North America, called Laurentia, was situated along the equator. "Vermont" was located right on the shoreline of this landmass, with a shallow tropical sea called Iapetus extending to the south and east. To the north and west are the eroding remains of an old mountain range. These mountains, called the Grenvilles, are the precursor to today's Adirondack Mountains. Sediments eroding from these mountains into the Iapetus Ocean helped form thick sedimentary layers in that basin.
Map from www.scotese.com
Layers of sediment deposited in the Iapetus would eventually become the sedimentary rocks seen today in the Champlain Valley. These rocks also preserved fossils of early marine life forms, including trilobites, corals, and brachiopods. The oldest known fossil reef in the world can still be seen in limestone beds outcropping in the Champlain Islands of northern Vermont.
500-400 million years ago
During this time, several crustal plates began to move together, forming new mountain ranges as the rocks collide and compress. During this process of mountain-building, known as the Taconic Orogeny, a chain of volcanic islands formed off the southeastern coast of Laurentia as oceanic plates collided (labeled as "Avalonia" on the map at right). Eventually these islands collided with Laurentia itself, further uplifting the mountains and causing a great deal of faulting and deformation in the sedimentary rocks caught between the two land masses. The sediments and rocks at the heart of the collision were transformed into metamorphic rocks such as schist, which make up most of today's Green Mountains. At this time, these mountains were much higher than they are today, perhaps comparable to the Andes or Himalaya.
Map from www.scotese.com
As the crustal plates collided, the compressed sedimentary rocks began to rise. Several major faults were formed, where entire blocks of rock thousands of feet thick were thrust up over one another. The greatest of these, the Champlain Thrust, created a long chain of hills that are still clearly visible along the eastern shore of modern Lake Champlain, with the main metamorphic rocks of the Green Mountains to the east.
400 - 25 million years ago
By this time, the basic structure of the Champlain Valley had developed, with the newly-formed mountains to the east, much older mountains to the west, and some form of valley in the middle. Although further collisions continue to form mountain ranges to the east (the Whites) and south (the rest of the Appalachians), little geologic history is preserved in the proto-Champlain Valley. The mountains on both sides probably continued to erode, but with no ocean basins to capture the sediments and record these events, the sediments were probably transported by rivers to further locations where they cannot be traced or interpreted as easily.
Map from www.scotese.com
"Vermont" and the proto-Champlain Valley were isolated within the continent, away from oceans, for hundreds of millions of years. At some point, though the timing is unsure, the land to the west began to uplift again, developing the steeper topography of today's Adirondack Mountains. The exact nature of this uplift is unknown, but it is still continuing today.
30,000 - 15,000 years ago
Many periods of glacial activity have affected North America over the last several hundred thousand years. During the most recent glacial period, thick sheets of ice extended as far south as Pennsylvania, covering Vermont to depths of over a mile. The movements of these great masses of ice helped shape the modern topograph of northern North America, grinding down mountains, carving valleys and lakes, smoothing rock surfaces, and depositing thick layers of sediments eroded further north. The cross-section below illustrates what the Champlain Valley might have looked like during the glacial maximum.
Map from www.scotese.com
As the glaciers moved over Vermont and the Champlain Valley, they helped carve out the modern topography of the valley, including the present bed of Lake Champlain. The ice blunted the Green Mountains and Adirondacks and shaped the outcrops of sedimentary rocks seen today. At their peak, the glaciers would have completely covered everything in Vermont.
10,000 years ago
As the glaciers eventually retreated, they dammed the natural northward drainage of the Champlain Valley, creating lakes over portions of Vermont, especially in river valleys. The land had been so depressed by the weight of the ice that for a brief time, the Atlantic Ocean flooded the Champlain Valley through the St. Lawrence Seaway, before the land rebounded and restored a northward drainage. These extensive water bodies left a thick blanket of marine and freshwater clay across much of the Champlain Valley.
Today
Today, Lake Champlain divides the old metamorphic rocks of the Adirondacks from the faulted sedimentary rocks of the Champlain Valley, which eventually give way to the younger (compared to the Adirondacks) metamorphic rocks of the Green Mountains to the east The Champlain Thrust is clearly preserved along the lake shore, forming a series of irregular hills with blunt faces to the west, and gentle slopes to the east.The clays and sands left behind by the post-glacial lakes and seas help form the modern soils and environment of Shelburne.
Geologic History