Glaciers, geology, and meteor impact craters have always fascinated me.
Right now I live in New England, a landscape that was recently, geologically speaking, glaciated. It has been about 12,000 years since the large glaciers here melted, leaving nothing behind but water, exposed bedrock, and a rubble of loose stones of various size and mud made from pulverized stone. Plants raced for the freed nutrients and the forest wandered north with them.
Now Vermont is thickly forested up to all but the highest peaks. Despite this signs of that past glaciation are everywhere, from large landscape features to the fine scale distribution of plants across that landscape.
One of the most prominent glacial features is the mountain Camel’s Hump, which, though not tall, can appear to loom over Burlington despite being 20 miles away.
I think it’s the implacable power of a glacier that awes me. Large glaciers are otherworldly in a way that is difficult to convey. The best description I have heard was in southeast Alaska where the large ice-fields are said to be the home of the dead. I spent two months working with a small group of people on the ice and it was easy to see why people said that.
Camel’s Hump, the mountain in the distance is a little over 4000 feet tall. That’s a bit less than a mile. At the height of the last glaciation the ice was more than twice that thick, burying Camel’s Hump so completely that you would only be able to tell it was there by looking via radar, sonar, or gravity scans. If you were there, you would be standing more than a mile above the mountain top looking at a flat white plain stretching past the horizon in all directions.
That vast depth of ice weighed an enormous amount and moved under its own weight, slowly flowing across the landscape. Glaciers are continually melting and the streams refreeze elsewhere. Some of this water finds its way to the bottom of the glacier and refreezes in cracks in the underlying bedrock the glacier has just scoured clean.
Water expands when it freezes and fragments of the bedrock were broken free and held, frozen into the bottom of the glacier. These entrained rocks scour the bedrock, like a file on metal, wearing it way, grinding it up, and tearing it loose. The shapes a glacier carves the land into are extremely characteristic and have their own specific names; col, cirque, esker, moraine, drumlin, chatter mark, arête, roche moutonnée, and many more.
The glaciers that covered the New England landscape were large and the features they left are correspondingly large.
Camel’s Hump is a textbook example of a large roche moutonnée. Roche moutonnée are low, rounded humps of bedrock rock that are ground into their shape by the action of a glacier. I assume roche moutonnée owe their name, “sheep rocks”, to French and Swiss sheep herders who would have spent a lot of time looking at these low, rounded rocks while thinking about sheep.
The shape of a roche moutonnée shows the direction of glacier flow, and from Camel’s Hump you can see that the ice flowed from the north to the south, from right to left in the photo above.
The northern surface has been ground to a shallow angle, while the south side is steep and jagged, where the glacier ripped cathedral sized chunks of rock loose.
There is a clear vegetation line on the mountain, hardwoods lower down, evergreens above. In Vermont the vegetation line is due primarily to temperature, moisture, and elevation. In areas more recently deprived of their glaciers the bands of vegetation climbing the slopes provide an accurate means of dating how long it has been since the glacier left.
Nothing in nature is random, the glaciers don’t just carve their way willy-nilly across the land, their actions are directed by the bedrock they are so eagerly tearing asunder. Camel’s Hump was a mountain before the glaciers came, and the glacier could only modify what was there, not make something completely new.
Seen from the south side, where the freeze/thaw and continual motion of the glacier have exposed the mountain’s heart you can see the folded, pastry dough like layers of rock that make up Camel’s Hump and the Green Mountains.
The tree covered slope beneath the cliff is a a boulder field dropped in place as the glaciers melted, now colonized by spruce and moss.