Deposition Associated with Glaciation
The Glacial Conveyor
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| The glacial conveyor and the formation of lateral and medial moraines on glaciers. |
Glaciers can carry sediment of any size and, like a conveyor belt, transport it in the direction of flow (that is, toward the toe; figure above a). The sediment load either falls onto the surface of the glacier from bordering cliffs or gets plucked and lifted from the substrate and incorporated into the moving ice. Geologists refer to a pile of debris carried by or left by glaciers as a moraine. Sediment dropped on the glacier’s surface moves with the ice and becomes a stripe of debris. Stripes formed along the side edges of the glacier are lateral moraines. When a glacier melts, lateral moraines lie stranded along the side of the glacially carved valley, like bathtub rings. Where two valley glaciers merge, the debris constituting two lateral moraines merges to become a medial moraine, running as a stripe down the interior of the composite glacier (figure above b). Trunk glaciers created by the merging of many tributary glaciers contain several medial moraines. Sediment transported to a glacier’s toe by the glacial conveyor accumulates in a pile at the toe and builds up to form an end moraine.
Types of Glacial Sedimentary Deposits
Several different types of sediment can be deposited in glacial environments; all of these types together constitute glacial drift. The term dates from pre-Agassiz studies of glacial deposits, when geologists thought that the sediment had “drifted” into place during an immense flood. Specifically, glacial drift includes the following:
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| Sedimentation processes and products associated with glaciation. Glacial sediment is distinctive. |
- Till: Sediment transported by ice and deposited beneath, at the side, or at the toe of a glacier is called glacial till. Glacial till is unsorted because the solid ice of glaciers can carry clasts of all sizes (figure above a).
- Erratics: Glacial erratics (figure above b) are cobbles and boulders that have been dropped by a glacier. Some lie within or on till piles, and others rest on glacially polished surfaces.
- Glacial marine: Where a sediment-laden glacier flows into the sea, icebergs calve off the toe and raft clasts out to sea. As the icebergs melt, they drop the clasts. Sediment consisting of ice-rafted clasts mixed with marine sediment makes up glacial marine.
- Glacial outwash: Till deposited by a glacier at its toe may be picked up and transported by meltwater streams that sort the sediment. The clasts are deposited by a braided stream network to form a broad area of gravel and sandbars called an outwash plain. This sediment is known as glacial outwash (figure above c).
- Loess: When the warmer air above ice-free land beyond the toe of a glacier rises, the cold, denser air from above the glacier rushes in to take its place. A strong wind, called katabatic wind, therefore blows at the margin of a glacier. This wind picks up fine clay and silt and transports it away from the glacier’s toe. Where the winds die down, the sediment settles and forms a thick layer. This sediment, called loess, sticks together because of the electrical charges on clay flakes. Thus, steep escarpments can develop by erosion of loess deposits (figure above d).
- Glacial lake-bed sediment: Streams transport fine clasts, including rock flour, away from the glacial front. This sediment eventually settles in meltwater lakes, forming a layer of glacial lake-bed sediment that commonly contains varves. A varve is a pair of thin layers deposited during a single year. One layer consists of silt brought in during spring floods and the other of clay deposited in winter when the lake’s surface freezes over and the water is still (figure above e).
Depositional Landforms of Glacial Environments
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| The formation of depositional landforms associated with continental glaciers. |
Picture a group of hunters, dressed in reindeer skin, gazing southward from the crest of an ice cliff at the toe of a continental glacier in what is now southern Canada. It’s about 12,000 years ago, and the glacier has been receding for at least a millennium. The hunters would have been able to see a variety of landscape features, some formed by glacial erosion and some by deposition, due to moving ice and meltwater. We've already described erosional features, so now let’s focus on the depositional features of the landscape (figure above a, b).
From their vantage point, the hunters would probably have seen a few curving, hummocky ridges of sediment in the region between the glacier’s toe and the horizon. Each of these ridges is an end moraine, formed when the position of the glacier’s toe remained in the same location for a while. Ice keeps flowing to the toe, and like a giant conveyor belt, transports sediment to the toe. As the ice melts, this sediment accumulates to form a pile of till, and this pile comprises the end moraine. Geologists refer to the end moraine at the farthest limit of glaciation as the terminal moraine. In the northeastern United States, a large terminal moraine built up during the Pleistocene Ice Age this ridge of sediment now underlies Long Island, New York, and Cape Cod, Massachusetts (figure above c). When a glacier starts receding, it may stall several times the end moraines that form when a glacier stalls while receding are known as recessional moraines. The hummocky layer of till between end moraines is known as lodgment till or ground moraine. Since this till was deposited by moving ice, clasts within it may be aligned and scratched.
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| Knob-and-kettle topography and drumlins characterize some areas that were once glaciated. |
The hummocky surface of a moraine reflects both variations in the amount of sediment supplied by the ice and the development of kettle holes. A kettle hole is a roughly circular depression made when a block of ice that calved off the toe of a glacier became buried by till. When the block eventually melts, it leaves behind a depression (figure above a, b). Geologists refer to a land surface spotted with many kettle holes separated by rounded hills or ridges of sediment as “knob-and-kettle topography” (figure above c).
In some locations, glacial ice flow molds underlying till into an elongate hill known as a drumlin (from the Gaelic word for small hill or ridge). Drumlins commonly occur in swarms, and tend to be about 50 m high. Their long axis trends parallel to the flow direction of the glacier. Notably, drumlins taper in the direction of flow a drumlin’s upstream end is steeper than its downstream end (figure above d, e).
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| Eskers are snake-like ridges of sand and gravel that form when sediment fills meltwater tunnels at the base of a glacier. |
As we've noted, not all of the sediment or “drift” associated with glacial landscapes was deposited directly by ice, for meltwater also carries and deposits sediment. Water transported sediment, in contrast to till, tends to be sorted and stratified. Sediment deposited in meltwater tunnels beneath a glacier itself may remain as a sinuous ridge, known as an esker, when the glacier melts away (figure above a, b). Braided meltwater streams that flow beyond the end of a glacier deposit layers of sand and gravel that underlie glacial outwash plains. Meltwater may collect in a lake adjacent to the glacier’s toe, to form an ice-margin lake. Additional lakes and swamps may form in low areas on the ground moraine. Sediments deposited in eskers and glacial outwash plains serve as important sources of sand and gravel for construction, and the fine sediment of former glacial lake beds evolves into fertile soil for agriculture.
Credits: Stephen Marshak (Essentials of Geology)
