I remember reading these books in my first semester of graduate school, before I had spent any appreciable amount of time out in the field, and the skeptic part of me immediately wanted the evidence that would support the stories I was being told. Not that I didn’t believe that there were glaciers, but I wanted to know how these authors/naturalists came to know these story. What did they find and where can I go to see what they saw? Over the past 10 years in Vermont I’ve discovered and been shown many of these special places that hold clues to the past. And as a teacher, I try and bring students to these places to see the evidence that might convince them that the stories are more than just words written in a page of a book, that they are stories written into the rock itself.
One fundamental principal in interpreting the past is this idea of uniformitarianism (something a young earth creationist would complain was not observational science but historical science; for more see this debatete with Ken Hamm and Bill Nye), essentially the idea that the present is key to understanding the past. As scientists, we assume this to be true, that the natural processes and laws that we observe today were present and operated the same way in the past.
The story of glaciers in Vermont
The story goes something like this: 110,000 years ago the climate began to cool (the most recent glacial period in an Ice Age that spans the last 2.6 million years). Year after year the glaciers advanced, on the east coast extending as far south to Long Island and down just south of to the northern border of the US out west. They reached their maximum extent 25,000 years ago, at which point the climate changed (the Barents Sea near Norway/Russia burped up some epic methane burps – article) and the glaciers began to retreat. As they retreated, the melt water formed large bodies of fresh water throughout the state, including Glacial Lake Vermont (13,500-12,000 years ago), which filled the Champlain Basin to about 620′ in elevation. Eventually, the Laurentide ice sheet had retreated far enough north exposing the basin to the St Lawrence seaway, allowing the fresh water to drain out (in a matter of hours) and salt water to slowly flow in, mixing in with the remaining water, giving rise to the Champlain Sea was born. The Sea filled the basin to about 320′ with salt water, but by about 10,000 years ago the Sea had reverted to a fresh water body of water, what we know and love as Lake Champlain (more on this later).
Evidence = key to a good story
A neat little story, easy to make notecards from to study for a quiz on dates and names. So how do we know this story to be true. Well the story isn’t necessarily true, but it is the best working theory that explains the oddball features dotting Vermont’s landscape (and New England, Canada, Scotland, northern Russia, etc etc). Going back to that idea of uniformitarianism, we can apply this to the glacial story of Vermont. If we visit the places where today glaciers are actively shaping the landscape we can look for the different ways in which they shape the land as they both advance and retreat. We can then go back to Vermont and see if we find remnants of these features. The key to the past is indeed in the present.
The evidence from an ADVANCING glacier
Regardless of where on earth we are, in places where there are active glaciers that are advancing (still growing/getting bigger and pushing ice forward at the margins), we find the same features repeated again and again. A glacier is a glacier is a glacier. And so each year as more snow accumulates on top of the glacier it adds more weight, and the pressure from this weight pushes outward on the body of ice. The glacier becomes mobile at its edges and actually flows over the landscape. As it moves it transforms the land leaving behind as evidence of its passage:
- Roche moutonnes
- Glacial valleys – U-shaped
- Till, moraines, glacial erratics
The evidence from an RETREATING glacier
All good things must come to an end, and as surely as they came, the glaciers left us. And in their wake they left of a whole host of new evidence from which we can reconstruct the past! As glaciers melt, all that water has to go somewhere. A retreating glacier occurs in areas where the ice melts quicker than it is replaced. There are only a handful of glaciers that today are still advancing due to local conditions. The rest of the planet’s glaciers are retreating. Here’s a short list of some features left behind as the glaciers retreated:
- Kettle ponds
- Kames + Kame terraces
- Drop stones
- Glacial Lake Vermont
- Champlain Sea
In the next few posts I’ll describe these features and describe where in the Burlington area you can find these features.
Relict populations & other evidence
** I should also note that there are other features that can be indirect evidence of a glaciers presence, like beavers. If you go to the Ruby Mountains of Nevada and climb up one of the glacially carved valleys you might get startled by a beaver slapping its tail. When you get to the summit of one of the peaks and look out, all you see is desert. I have this funny image in my head of a beaver carrying a suitcase busting at its seams, full of aspen twigs, walking across the desert in search of water. A better – or at least more accurate story – would be that with the glaciers extending so much farther south than they do today, the climate across the US was much different than it was today. Much of that Nevada desert land was far less arid than it is today. With an abundance of rain, beavers would have easily followed river ways that connected different habits. As the glaciers retreated and Nevada became a desert, relict populations of beavers were stranded in high elevation, wet environments.