Published March 25, 2021 by the Vermont Community Newspaper Group in The Citizen
Q: How does the Champlain Valley connect to the ancient supercontinent Pangea?
A: I climbed to the top of Mt. Philo to see the magnificent view of the Champlain Valley from the top of the cliff face – the pastoral farmland in the foreground, the beautiful lake stretching north and south, and the Adirondacks on the horizon. But I also wanted to get an overview of something much, much bigger. From the top of Mt. Philo, the evidence of two scenes of an ancient global-scale drama can be seen: the formation and break-up of the supercontinent Pangea.
The first scene in the drama took place literally under my feet. As I stood there, the cliff rocks seem firmly anchored in place. But hundreds of millions of years ago they had been in motion, pushed by the immense forces of colliding crustal plates as the supercontinent Pangea was forming. As the African and North American plates were moving shoulder to shoulder, the enormous compression folded some of Vermont’s bedrock into the Green Mountains and northwest Africa’s bedrock into the Little Atlas Mountains.
Further west of the Green Mountains, some of the bedrock fractured and slid as it was pushed upward and westward over stationary rock. The cliff face I was standing on was the sharp leading edge of one of those moving rock faces that had been thrust many miles westward. In fact, it is part of the westernmost fault line in Vermont created by Pangea’s formation that extends the length of Lake Champlain (called the Champlain Thrust Fault by geologists). Here at the park, it’s about 500 feet higher than the park entrance, even after millions of years of erosion. The pastoral lowlands I can see below me were the stationary rocks that the cliff rocks had over-ridden. When I’m standing on this abrupt cliff, I’m on the westernmost line in Vermont where the extensive, prolonged movement of supercontinent Pangea’s formation ground to a halt.
After Pangea formed, it stayed together as one supercontinent for many millions of years. During that time, the major geological forces in the area I was looking at were quite quiet.
Then, about 200 million years ago, Pangea started to pull apart and break up into the separate continents we know today. The next scene in the world-scale drama played out in the panorama I was seeing beyond the farmlands out in the long, blue ribbon of Lake Champlain. The lake water’s smooth surface conceals the shape of the land under it. But the lake basin’s shape was created by Pangea coming apart.
I could see the Charlotte Ferry route to Essex, NY from the park cliff. When crossing on the 3-mile route from the Vermont side, the lake bottom drops off steadily over the first 2/3 mile to 100 feet deep, then in the next 1/8 mile it suddenly drops to 300 feet deep, then it quickly bottoms out at about 400 feet deep. It is relatively flat mid-lake at the 400-foot depth, then it rises fairly steeply again as you near the New York shore. Scientists who have studied the lake say there are about 300 feet of loose sediment below the current lake bottom. That makes the depth to the bedrock bottom about 700’ below lake level. When I ferry across the lake, I’m riding over a lake basin that is shaped like a very deep, narrow trough with steep sides. How did this form?
When the supercontinent Pangea started to pull apart, the bedrock started to stretch. In one of the stretching sessions, the bedrock where the lake is now broke on both sides creating a large block. The large bedrock block fell down relative to the surrounding rock, creating the basin. The basin eventually filled with water and formed Lake Champlain. One way to think of the lake basin is that it is one of the stretch marks of Pangea breaking up.
While I contemplate the view from the park cliff, I know that the supercontinent Pangea was responsible for the major highs and lows of the Vermont landscape before me. Pangea’s forces pushed up the cliff I was standing on as the continental plates squeezed together (now about 500’ up from the park’s front gate), and pulled open the lake’s basin as it was coming apart (now more than 400’ below the shoreline). My magnificent view was shaped by some very large-scale drama, indeed!
Copyright 2021, Jane Dorney