landforms from glacial erosion and deposition
Melting glaciers deposit all the big and small bits of rocky material they are carrying in a pile. These unsorted deposits of rock are called glacial till. Glacial till is found in different types of deposits. Linear rock deposits are called moraines. Geologists study moraines to figure out how far glaciers extended and how long it took them to melt away. Moraines are named by their location relative to the glacier: Lateral moraines form at the edges of the glacier as material drops onto the glacier from erosion of the valley walls. Medial moraines form where the lateral moraines of two tributary glaciers join together in the middle of a larger glacier (Figure 1.7). Ground moraines forms from sediments that were beneath the glacier and left behind after the glacier melts. Ground moraine sediments contribute to the fertile transported soils in many regions. Terminal moraines are long ridges of till left at the furthest point the glacier reached. End moraines are deposited where the glacier stopped for a long enough period to create a rocky ridge as it retreated. Long Island in New York is formed by two end moraines.
Rocks carried by a glacier are eventually dropped. These glacial erratics are noticeable because they are a different rock type from the surrounding bedrock.
depositional features of glaciers
As glaciers flow, mechanical weathering loosens rock on the valley walls, which falls as debris on the glacier. Glaciers can carry rock of any size, from giant boulders to silt (Figure 1.6). These rocks can be carried for many kilometers for many years.
Glaciers erode the underlying rock by abrasion and plucking. Glacial meltwater seeps into cracks of the underlying rock. When the water freezes, it pushes pieces of rock outward. The rock is then plucked out and carried away by the flowing ice of the moving glacier (Figure 1.1). With the weight of the ice over them, these rocks can scratch deeply into the underlying bedrock, making long, parallel grooves in the bedrock, called glacial striations. Mountain glaciers leave behind unique erosional features. When a glacier cuts through a V-shaped river valley, the glacier plucks rocks from the sides and bottom. This widens the valley and steepens the walls, making a U-shaped valley (Figure 1.2). Smaller tributary glaciers, like tributary streams, flow into the main glacier in their own shallower U-shaped valleys. A hanging valley forms where the main glacier cuts off a tributary glacier and creates a cliff. Streams plunge over the cliff to create waterfalls (Figure 1.3). Up high on a mountain, where a glacier originates, rocks are pulled away from valley walls. Some of the resulting erosional features are shown in Figure 1.4 and Figure 1.5. Glacial striations point the direction a glacier has gone. A U-shaped valley in Glacier National Park. Click image to the left or use the URL below. URL: Yosemite Valley is known for waterfalls that plunge from hanging valleys. (a) A bowl-shaped cirque in Glacier Na- tional Park was carved by glaciers. (b) A high altitude lake, called a tarn, forms from meltwater trapped in the cirque. (c) Several cirques from glaciers flowing in different directions from a mountain peak, leave behind a sharp sided horn, like the Matterhorn in Switzerland. (d) When glaciers move down opposite sides of a mountain, a sharp edged ridge, called an arte, forms between them. Snowmelt and melting glaciers combine to create a fast moving stream at Glacier National Park.
Several types of stratified deposits form in glacial regions but are not formed directly by the ice. Varves form where lakes are covered by ice in the winter. Dark, fine-grained clays sink to the bottom in winter, but melting ice in spring brings running water that deposits lighter colored sands. Each alternating dark/light layer represents one year of deposits. (a) An esker is a winding ridge of sand and gravel deposited under a glacier by a stream of meltwater. (b) A drumlin is an asymmetrical hill made of sediments that points in the direction the ice moved. Usually drumlins are found in groups called drumlin fields. Click image to the left or use the URL below. URL:
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long ridges of material dumped at the furthest point the glacier reached.
a) end moraines b) lateral moraines c) ground moraines --> d) terminal moraines
on a mountain, a glacier originates in a
a) horn b) arete --> c) cirque d) coll
unsorted deposits of rock are called a glacial ____.
a) erratics --> b) till c) drumlin d) valley
a large rock that was dumped by a glacier, sometimes far from where it originated is a
a) glacial striations --> b) glacial erratic c) glacial till d) glacial moraine
glaciers can carry rocks of any size.
--> a) true b) false
varves display an annual cycle of dark, fine clays deposited in winter and lighter sands deposited in spring.
--> a) true b) false
the water falls in yosemite are created by water falling down from a
a) cirque. b) tributary valley. c) v-shaped valley. --> d) hanging valley.
a ridge of bedrock carved by a glacier is a(n) _; a ridge of till deposited by a stream beneath a glacier is a(n) _.
--> a) arte; esker b) esker; arte c) horn; lateral moraine d) lateral moraine; horn
how do glaciers erode underlying rocks?
a) glacial striations. b) saltation. --> c) abrasion and plucking. d) ice wedging.
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