Glacier Tracks
A glacier frozen to the rock beneath does not erode much. However, thawed-bed glaciers, especially those with surface meltwater streams draining to their beds through holes (something like cave passages, although formed in different ways), can erode even more rapidly than streams or wind erode, creating features made only by glaciers. Consider for a moment the Great Lakes of the U.S. and Canada - these lakes were carved by glaciers. The bedrock beneath Lakes Superior and Michigan is well below sea level, and was carved by glaciers, not rivers! Today, rivers carry sediment into the Great Lakes, slowly filling them up. We will see later that over the last million years, times when the glaciers were eroding have alternated with times when streams were filling the lakes back up with sediment, and the streams have had more filling-up time than the glaciers had eroding time. And yet, there are the lakes, not the least bit full of sediment. The glaciers have been much better at their “job” than the streams. The same can be said for many other places. It is not too extreme to say that the regions that had glaciers 20,000 years ago and are free of ice today still preserve a glacial landscape.
Ice moving over bedrock “plucks” smaller rocks free, then uses those rocks to abrade or “sandpaper” the bedrock, scratching and polishing it. As ice flows over a bedrock bump, the side the ice reaches first is abraded smooth while the other side is plucked rough, as you can see so clearly on slide 15 of the Yosemite VTrip earlier in this module. Subglacial streams sweep away the loose pieces and may cut into the rock.
Plucked and abraded rocks show clearly that glaciers were present, but so do big features, such as Yosemite Valley. The steep walls and nearly flat bottom of the valley make a characteristic “U” shape. A river without a glacier tends to cut downward and then mass movement processes remove material from the walls, giving a "V' shape. (Where a V-shaped stream enters an ocean to make a delta, the outward and upward growth of the delta over time may eventually fill the bottom of the V with mud to make a flood plain, as we saw with the Mississippi, but initially, when a stream is cutting down, it tends to make a V.) However, glaciers are quite wide and can erode across a broad region, giving the classic "U" shape.
Yosemite is famous for its waterfalls from “hanging valleys” far up in the cliffs, another sign of recent work by glaciers. With streams, steeper ones generally erode faster. If a main river cuts down rapidly, its tributary streams become steeper and cut downward faster. In this way, even a small side stream can “keep up” with the erosion by the main stream so stream erosion generally produces "rapids" rather than waterfalls. Glaciers are different. A main glacier often fills its valley, burying most or all of the rock. The ice from a side glacier then does not drop steeply down into the main glacier. For various reasons, the main glacier tends to erode faster than the side glaciers. When the ice melts, a “hanging valley” is left behind — a small stream that replaces the small side glacier must plunge over a glacially carved cliff and then flow across the bottom of the “U”-shaped valley to reach the main stream. Eventually, the side stream will wear away the waterfall. But today in Yosemite, numerous streams emerge from small “U”-shaped hanging valleys to cascade down the glacially carved cliffs—the landscape is pretty much what the glaciers left. (Piles of rocks at the bottoms of waterfalls show that the streams are indeed changing things, but slowly.)
Glaciers make many other erosional features. At the head of a glacier, freezing and thawing can break rocks, and the loose pieces can be hauled away by the glacier, carving a bowl into the side of a mountain. If bowls chew into a mountain from opposite sides until they meet, a knife-edged ridge is left—such as the Garden Wall of the continental divide in Glacier National Park, which we’ll meet soon. Where three or more bowls intersect from different sides, a pinnacle of rock is left, such as the Matterhorn of Switzerland. Mountaineers have dubbed the bowls cirques, the ridges aretes, and the pillars horns, and geologists continue to use these terms.
Glaciers also leave distinctive deposits. Streams, waves and wind all sort rocks by size, leaving too-big ones behind and carrying away smaller ones, eventually depositing sediments dominated by a single grain size such as sand. Glaciers don’t care how big the rocks are that the ice carries, so a deposit put down directly from the ice may have the tiniest clay particles mixed in with house-sized or bigger boulders. Such a deposit is called a glacial till. Till plus glacial outwash (sediment washed out of a glacier by meltwater) may be piled up together in a ridge that outlines the glacier, called a moraine.
Pennsylvania has a Moraine State Park, which features glacial moraines. Cape Cod is a moraine, and a moraine is draped across Long Island, showing some of the places where glaciers from the ice age ended.