Glacial Deposition


In general, the deposits associated with both alpine glaciers and continental ice sheets are similar. Both deposit their eroded debris in their downstream portions or as the melt back or "retreat" (Note, retreat refers to the front or outer edge of the ice, not to a change in the direction of movement. The glacier is still moving downward or outward by the pull of gravity and the weight of the overlying snow.) ALL GLACIAL DEPOSITS are DRIFT. Glaciers are powerful enough to carry tiny and huge rock debris, and when they drop it, the ice drops it indiscriminantly. Thus, material deposited by ice is unsorted or mixed in size. This non-sorted material is called TILL. ALL TILL IS DRIFT BUT NOT ALL DRIFT IS TILL. Water running off of the ice is MELTWATER, and that meltwater carries some of the rock debris away. Water sorts its load, so that which it drops is drift, but not till. If the water and the ice work together, the material is sorted, so it is drift but not till. Also, wind deposits associated with glaciation (loess) are drift but not till.

Landforms made of concentrated till are called MORAINES. Five different types of moraines are commonly recognized. A TERMINAL MORAINE forms when the glacier, acting like a conveyor belt, moves material to its outermost or lowermost end and dumps it. A terminal moraine marks the farthest advance of the ice. It is a rough, irregular series of hills composed of till. As the glacier melts back, it is still moving debris to its front and dropping it. That material then blankets the landscape as GROUND MORAINE. New England is noted for its many ERRATICS, rocks brought to the area by the glaciers, that farmers have removed from their fields and stacked as fence rows. If the climate is such that the glacier stabilizes and stays in one position for a time, the conveyor belt action will continue to dump the glacial load at the front as another ridge of material. Formed just like the TERMINAL MORAINE except that it is not at the farthest advance of the ice are the RECESSIONAL MORAINES. A glacier will have only one terminal moraine, but can have many recessional moraines. Part of the load of the glacier is concentrated on its sides. This material, and the deposit of it with the melting of the ice, is called the LATERAL MORAINE. When two tributary glaciers merge, such as in our example in the formation of a hanging trough, the adjacent lateral moraines of each will merge and be trapped between the two masses of ice. When the ice melts, the ridge of till that is dropped onto the landscape is called a MEDIAL MORAINE (for a continental ice sheet, these are INTERLOBATE MORAINES).

One till deposit is not classed as a moraine. These are DRUMLINS. Drumlins are formed in groups underneath the ice. They are streamlined hills molded by the flow of the ice. Shaped like an overturned bowl of a spoon, the lower, narrower end points in the direction that the glacier was flowing.

Three types of deposits are drift but not till. Outwash deposits are made by the meltwater flowing away from the ice. A broad, stratified (layered or sorted), gently sloping surface is an OUTWASH PLAIN. The wind can pick up and blow the ground up rock flour, distributing it over wide areas. This material is LOESS.

When the glaciers and meltwater act together, the third type of deposits are ice-contact stratified drift. This means that the ice was in contact with the deposition, but water was sorting the drift. Ice-contact stratified drift composes three very important types of glacial landforms. A stream in the ice will have debris in its bottom just like a stream on land. When the ice melts, this material is dropped down to the surface. It can form a snake-like ridge of material added to the landscape that is called an ESKER. A hole in the glacier will have water flow into it, and that water will carry debris with it. When the glacier melts, this pile of debris is dropped onto the landscape as a KAME. Kames are low, cone shaped, steep-sided hills. A KETTLE forms where a block of ice breaks off of the glacier and is carried away by the meltwater. It then lodges in the debris and may be buried while material fills in around it. When the ice melts, its absence results in a surface depression that may or may not retain water. It is not unusual to find kames, kettles, and eskers together since the same type of glacial conditions promote their formation.

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