Touring Bottom of Glacial Lake Hitchcock With Dartmouth Grad Students
Justin Stroup and Laura Levy are doctoral students in the Earth Sciences Department at Dartmouth College. (Valley News - Jennifer Hauck) Purchase photo reprints »
Hanover — Going for a guided history tour with Laura Levy and Justin Stroup is a time-expanding experience.
These two doctoral students in the Earth Sciences Department at Dartmouth College don’t think in terms of what happened 100 or 200 years ago. They think in geological terms, reaching back into history for tens of thousands of years as they point out landforms that were deposited in the Upper Valley when a huge mass of ice known as Glacial Lake Hitchcock receded from the area during the last Ice Age.
Both Stroup and Levy are fourth-year graduate students at Dartmouth, working under the guidance of Meredith Kelly. Both are studying how glaciers have responded to climate change over the last 10,000 years.
But while Levy heads north to Greenland for field research each summer, Stroup heads south to Peru. Levy, 36, and Stroup, 28, recently met with the Valley News for a one-hour hike along the edge of the Hanover Country Club golf course. Starting from the Dartmouth Outing Club on Occom Pond, they struck out on the trails in Pine Park Forest. What follows is an edited transcript of the conversation that unfolded as they hiked through the woods at the edge of the golf course.
Valley News: What is it about the nature of glaciers that the two of you find compelling?
Laura Levy: I don’t know specifically what it is. But being in Greenland in the summer in some of the most pristine environments in the world and seeing landforms that have changed over time is somehow really compelling. It makes you want to go back every summer. People think it’s crazy that you want to go to the Arctic every summer, but it’s a really amazing place and it really just draws you back year after year.
Justin Stroup: I really love being up in the high mountains and seeing both the glaciers and the glacial environment and being someplace that’s really, really amazing, and where you can … think about … how the world connects.
Even though I work in the tropics and Laura works (at) the pole, we spend a lot of time trying to understand how you can understand climate change in the global context. So we spend time looking at records from all over the world to piece together what we think happened in the past.
And we use that to inform modeling to understand past and present and future climate changes.
VN: Right now we’re on a golf course in Hanover, so that’s what I see.
What do you two see?
LL: Yesterday I went shopping with a friend and we’re driving across this beautiful landscape. She was looking at the fall foliage and I was wondering why there was so much sand in the area and where the sand was coming from and what kind of glacial deposits we were looking at. We definitely see the world from a slightly different lens.
JS: The other day, I went for a bike ride way up in Vermont and … every place there was a little landslide or a little exposure, I was craning my head as I rode by.
LL: I know this map doesn’t convey to the recording, but let me give you a visual. … We’re right here near White River Junction, and the ice (of Glacial Lake Hitchcock) sat here about 14,200 years ago. So a lot of the surface features you see in the Connecticut River Valley are from Glacial Lake Hitchcock, … which filled in the valley. … So a point of reference is the Dartmouth green. That really nice flat area is actually glacial lake bottom. You can imagine that the lake itself, when it was full, was about to the top of Baker Berry Tower, about 115 feet. We would still be under about 50 feet of water standing here (on the golf course).
VN: But the golf course is obviously a manmade landform.
JS: Only some of it is. … The feature we’re standing on right now, this hill … was formed as water underneath the glacier formed … a channel, a river under the ice. That river filled up with gravel and as the ice retreats, that filled river of gravel is left behind and it makes this long wiggly shape all the way down the valley. The feature is called an esker. … We’re walking on the top of the esker right now. … If you look off to the left and the right, where it’s really steep, you can see that this was just a huge channel … underneath the ice, that water is gushing through with sediment and that sediment is lodging and filling in this channel. This landform is made of gravels, like cobbles, which are basketball sized (or smaller).
LL: They’re all very rounded, which is indicative of being deposited and eroded in a river.
JS: Eskers are usually pretty long features. So you can trace this (esker) from White River up into Vermont … for quite a ways.
LL: Now we’re coming down off the side of (the esker). (Holds up a smooth, melon-sized rock.) Some of the evidence that this landform was deposited under water are these really rounded cobbles. … If we had a bulldozer, which is what all sedimentary geologists want … and we could dig into the side of this hill, we would see a lot more cobbles that are really rounded and they’d be all different sizes.
VN: All of this that we’re walking on right now, these are all cobbles deposited by a glacier?
LL: Yeah. Exactly.
VN: I would have thought this area was younger than 14,000 years ago because of all the trees.
LL: It’s still part of the esker. This top layer is soil development that has developed over probably the last few thousand years and the trees have grown out of that. But the actual, large-scale feature that they’re sitting on is still the esker. … There are other interesting landforms (in the area). … The Lebanon airport is actually built on an old delta top … from the river that used to feed into Glacial Lake Hitchcock. … (The group approaches and crosses a stream.) JS: When you come into places where there are streams, streams erode and down cut through the landscape and what happens is that as they down cut … you can get exposures to see the sediments … LL: … without the bulldozer.
JS: Without the bulldozer. Right. The stream does the bulldozing and we search around and find these exposures. So we’ll go see a couple of those. … (They shovel a chunk of soil from the side of the stream.) If you look in just the right light, you see the layers. These are (called) varves, those (glacial) deposits that are annual. They have a coarse layer, … the lighter colored stuff, and that happens during the summer.
And then you move up and there’s this darker clay layer and that’s the winter layer. These vary in scale in the Connecticut River Valley, from things about 3 feet in thickness to these really fine (samples) that are a centimeter to millimeter scale.
VN: So this varve is showing us literally one year after another, similar to tree rings?
LL: Yes. This is about 15 years. It’s very similar to trees. And just like tree rings we can learn about the past environment by looking at these. If you have a storm event, you have a whole bunch of debris washed into the lake that will be deposited into the varve. So sometimes you have a very sandy, coarser layer and that’s indicative of a storm event.
VN: It’s strange to think that the Dartmouth green was once the bottom of a lake, LL: I believe the Lebanon green, the Lyme green, are also the same. A lot of the cemeteries in the area are built on small deltas that were deposited in Glacial Lake Hitchcock, because they’re well-drained.
Generally in this area, when you can find a flat area … it probably is a lake bottom.
VN: Pine Park itself is a beautiful area.
JS: We bring students back here for teaching sometimes. It’s a good place to show some really neat and close-by geology. It’s tangible, too, because you can imagine this big lake and think about the sediment being put down in these annual layers and that that can tell you how long the lake has been there. It’s a nice natural laboratory.
Diane Taylor can be reached at 603-727-3221 or firstname.lastname@example.org.