Time Scavengers

Adriane here-

Every Semester, the University of Massachusetts Amherst Department of Geosciences offers the class Introduction to Geology. The course is designed for undergraduate students who need science credits for their degree, but it is also a required course for our geoscience undergraduate majors. The course has a lab that complements and expands on topics that are covered in lecture, such as  plate tectonics, igneous, metamorphic, and sedimentary rocks, river dynamics, topography, and the history of how the Connecticut river valley was formed. The labs are run by four to five teaching assistants, graduate students who are pursuing their master’s or PhD degrees. We take the students on two field trips as part of their labs: the first trip is to look at geologic features in the valley leftover from the glaciers, and the second to look at the major rock formations in the valley.

This post is about the glaciation that ended around 20,000 years ago, and the imprint the ice and ice melt left on this area of western Massachusetts. This time of huge ice sheets that covered northern North America, Europe, and Asia is referred to as the Last Glacial Maximum. In North America, the glaciation is referred to as the Wisconsin Glaciation. The glaciers that covered North America reached their maximum southern extent about 26,000 years ago, after which the ice began to melt and retreat back to the north. At the glacier’s maximum extent, Massachusetts was totally covered by ice. Estimates based on how thick the ice was range from 1 to 2 kilometers  (0.62-1.24 miles)! That’s a ton of ice! Because ice is very heavy, it depressed the Earth’s crust below it. In southern New England, the ice is estimated to have depressed the Earth’s crust down by about 50 meters (~165 feet; Oakley and Boothroyd, 2012). Once the ice melted, the crust of the Earth began to pop back up. This phenomenon is called isostatic rebound. Satellites that measure the rate of isostatic rebound indicate that parts of Greenland and Canada, where the ice was thickest, are still popping back up today.

Back at a more local scale, there are several features around UMass Amherst that we, the graduate students that teach the Introduction to Geology labs, can identify that were created as a direct result of the glaciers in this area. I’m going to take you on our glacial field trip virtually, so that you, too, can get a first-hand look at the types of geologic features the glaciers left behind! First, a fun aside: There are several large boulders all over campus that, at first glance, look like they were placed in specific locations for an aesthetic affect. Upon closer inspection, the boulders are made from a certain rock type that occurs in our valley. Glaciers, and ice, act as bulldozers, and have no problem picking up and carrying huge chunks of rocks. These boulders, then, were picked up from the nearby hills and deposited all over the valley, with some ending up on our campus! These boulders aren’t part of our field trip, but are neat nonetheless.

The first stop on our field trip is right on campus, behind our football stadium. Here, there is a small creek with about 5 feet of the upper part of the soil profile exposed. But there’s something special about the soil here: it is unlike anything found in other places of the world: varves! Varves are alternating bands of dark and light-colored clay layers that made up the bottom of a lake that used to cover the valley. The lake was created once the glaciers began to melt, leaving more parts of Massachusetts exposed. The meltwater from the glaciers flowed via river into the Connecticut River Valley, and became dammed here. This created Glacial Lake Hitchcock. The varves are remnants of the sediment that collected on the bottom of this lake. Varves are awesome because they record climate changes on a seasonal basis! The dark bands contain finer clay particles, and are deposited during the winter months when there is less sediment being brought into the lake by the river (during the winter months, there is less glacial melt, and thus less water flowing in the rivers). Lighter varve bands are usually made of coarser (or larger) grains and are deposited during the spring and summer months when glacial melting increases, bringing more water and thus sediments into the lake. By counting the pairs of dark and light varves, scientists can estimate how old Glacial Lake Hitchcock was. Varves from the lake were counted by scientists at UMass Amherst, and they found that the lake was around for at least 4,000 years, from 17,5000 to 13,5000 year ago (Rittenour et al., 2000)!

After checking out the varves, our second stop is a kettle hole a few miles from campus. A kettle hole is a depression in the Earth formed from a chunk of ice that broke off from the retreating glacier. The ice chunks become buried by glacial outwash, or the mix of water and sediment that spreads across the land as the glacier melts. Thus, the ice chunk is completely buried by sediments. After some time, the chunk also melts, which then creates the kettle hole. These features are prominent throughout New England, and are usually small in size.

The third stop on our glacial field trip is to the Sunderland Delta. A delta is a place where a river meets a larger body of water, like a large lake, ocean, or sea. Some rivers flow fast, and some flow slower. In general, the faster a river flows, the more sediment it can move and carry. I mentioned earlier that the glacier began to melt back, and that melt water was transported by a river into Glacial Lake Hitchcock. The river was quite large, and had the ability to move sand-sized sediment. But where the river met the calm waters of the lake, it lost its velocity, and thus its ability to carry sediment. The sediment was then ‘dropped’ at the mouth of the river where it emptied into Glacial Lake Hitchcock. This dropped sediment formed a delta, or an area where fine-grained sand was deposited. This sand accumulated over thousands of years. Today, these sand deposits that make up the delta are mined by humans for use in concrete and manufacturing. There are several open mining pits around the university, but one in particular preserves the features of the delta, namely topset and foreset beds of sand. When the river gets close to the larger body of water, it begins to slow down. The loss of velocity leads to the river dropping some of its sediment it is transporting. This sediment is laid down in thin sheets that lie flat. These sediments form topset beds. Where the river meets the body of water, it is slowed even more, and the rest of the sediment it carries is dropped. These particles form a slope down into the lake, and make up the sloping foreset beds.

Once the students understand how the Sunderland Delta was formed, we then move downhill to our fourth stop: a trout hatchery right down the road. This, by far, is the coolest stop, as it contains a unique geologic feature: a natural spring! At our first stop, you saw that the base of Lake Hitchcock was composed of very fine sediment called clay and silt. Clay and silt grains are flat, and when they are compressed over time, they don’t allow water to pass through very quickly. In the introduction paragraphs of this post, I also explained how the Earth’s crust underneath New England, including Massachusetts, popped back up, or rebounded, after the overlying weight of the glaciers was gone. When the land began to rebound, that caused the clay layers of the lake to crack. When this happened, the groundwater that was stored deep in the sediment under the clay was able to come to the surface. Here, at the trout hatchery, is one of the places the groundwater is able to come to the surface via cracks and conduits in the thick clay layers! It can be seen bubbling to the surface continually throughout the year. The video below shows the spring in action:

The water stays a constant temperature year round because it originates from so deep within the sediment. The constant temperature and clarity of the water is great for raising trout, because the water rarely, if ever, freezes during the winter and is never too hot during summer months! The trout that are raised at the hatchery are released into local streams and rivers so that fishers do not over-fish the local populations. This stop is one of my favorites, as it is an excellent example of how geology and biology go hand-in-hand, and how the geologic processes of the past are relevant and useful today.

Our fifth and final stop is just down the road from the trout hatchery. The feature here is not as impressive or obvious after the large delta feature and the natural spring, but it records an important phenomenon related to the retreat of the glaciers nonetheless. On the side of the road is a small hill that most local folks pass by probably everyday. This hill is covered in trees and vegetation, and one might totally overlook it quite easily. But if you stop and dig down 4-5 inches through the roots and topsoil, you’ll hit sand!  This small hill is, in fact, a sand dune that was formed from winds towards the end of the Last Glacial Maximum.

When the glacier that covered Massachusetts began to melt back, the Earth was beginning to warm up. The area to the south of Massachusetts was becoming warmer, but on top of the glacier to the north, the air temperature was still very cold. This difference in air temperature, or temperature gradient, created strong winds that blew from the warmer regions towards the colder regions. This phenomenon happens today at the beach: during the day, the wind blows towards the ocean from the hot land; but at night, the wind direction changes as the land cools down until it is cooler than the ocean water. The beach is also characterized by sand dunes,which are the products of these strong winds depositing small sand grains behind the beach. Just like at the beach, the strong winds moving towards the glacier picked up small sand grains and deposited in the valley near UMass, where they are still visible today!

The features that we show our undergraduate students and that are explained here are just a few features in our valley that are leftover from the massive glaciers that once covered the land. All around New England, there is evidence of the heavy ice that was once here not too long ago: exposed rock where the glaciers scraped away soil; glacial striations, or scratches, in the exposed rocks from the glaciers moving over them; and potholes in the bedrock near the rivers, where melted water mixed with larger pebbles and boulders under the ice to carve out rounded holes in the rocks. If you’re ever in New England, keep your eyes peeled for evidence of the glaciers; it’s literally everywhere!

 

Citations

Oakley, B. A., and Boothroyd, J. C., 2012. Reconstructed topography of Southern New England prior to isostatic rebound with implications of total isostatic depression and relative sea level. Quaternary Research 79, 110-118.

Rittenour, T. M., Brigham-Grette, J., and Mann, M., 2000. El-Nino Like Teleconnections in New England during the Late Pleistocene. Science 288, 1039-1042.