Continental Glaciation
Unlike alpine glaciers which are restricted to valleys, continental glaciers advance along a wide front and cover large areas. As recently as 12,000 years ago continental glaciers cover much of the northern tier of states in the US. Some of the features that allow us to recognize areas that were glaciated in the past are illustrated on the topographic maps below.
The map at right is from Rhode Island. What evidence can you see that this area has been glaciated? Study this map than scroll down to see key features labeled.
Key observations you should make: Note the ridge that runs across the middle of the map (between the dashed blue lines). The ridge is fairly wide (about 1 km) and has a very irregular top. Note the complex pattern of the contours of the ridge and the numerous depression contours along it.Also note the swampy areas north of the ridge.
From the previous section on alpine glaciers you may have recognized the irregular ridge as a end moraine. As with alpine glaciation, end moraines are ridges made of till that were deposited from ice melting at the edge of a glacier. Since continental glaciers are much wider than Alpine glaciers, end moraines in continental glaciers tend to be much longer. Though it is not apparent from this map this ridge extends for many kilometers in both direction.The other feature on this map is called ground moraine. Ground Moraine is just till covered areas were the till is not pilled into a ridge. These till covered areas can be recognized by irregular topography and poor drainage. The large swampy areas suggest poor drainage, and their proximity to end moraine suggests a glacial origin.
The map at right is from Maine. Study the map. What evidence do you see for glaciation?Compare the ridge on this map to the end moraine on the previous map. What difference do you see? Do you think this is an end moraine, or is it something else?
The large swampy areas suggest poor drainage, mostly because they are covered by glacial till. (Note, not every swamp forms because of glacial deposits).The ridge on this map is very different than the end moraine on the previous map. This ridge is narrower with a more regular, narrow top. It is what is known as an esker.
An esker is a ridge produced from sediment that was carried by melt water on or through a glacier. This sediment is deposited on the ice just as sediment is deposited along a stream on the ground. However when a glacier melts the stream laid sediment on the ice drops to ground and forms a ridge. Notice the dimensions of the esker are similar to the unrelated stream to the east.
One other clue to the origin of this ridge is seen on this map. Notice the gravel pit along the ridge. Glacial till is very poorly sorted and contains lots of clay so is not a good source of gravel. The sediment deposited in eskers tends to be well sorted. The flowing water carries away the smaller particles leaving behind gravel. The gravel pit along this ridge suggests it is made of well sorted sediment so is most likely an esker.
Look at the ridges on this map from western New York. What pattern do you notice? Note the characteristic shape of these hills. What glacial feature do these hills represent?
The most obvious feature on this map are the series of long hills that are parallel to each other (outlined in red on the map). These hills all have a similar shape. They have steep slopes (note closely spaced contours) on the NW end of the hill and gentle slopes (more widely spaced contour lines) on the SE side.This shape is very characteristic of Drumlins. Drumlins are hills of till that form under glaciers. Drumlins often form in large groups, all with the same shape and all parallel to each other. The asymmetry of slope is interpreted to indicate the direction the ice flows. The steep slope at one of the narrow of the drumlin points in the direction the ice is flowing from. The gentle slop points in the direction the ice is flowing to. The blue arrow indicates the direction the was flowing when these drumlins formed.
Also note the swampy areas, a common feature in areas covered by glacial till.
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Last Updated: 03/11/2008
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