Glaciation

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Glaciation

The covering of the earth's surface with glacial ice. The term also includes the alteration of the surface of the earth by glacial erosion or deposition. Due to the passage of time, ice erosion can be almost unidentifiable; the weathering of hard rock surfaces often eliminates minor scratches and other evidence of such glacial activities as the carving of deep valleys. The evidence of deposition, known as depositional imprints, can vary. It may consist of specialized features a few meters above the surrounding terrain, or it may consist of ground materials several meters in thickness covering wide areas of the landscape.

Only 10% of the earth's surface is currently covered with glacial ice, but it is estimated that 30% had been covered with glacial ice at some time. During the last major glacial period, most of Europe and more than half of the North American continent were covered with ice. The glacial ice of modern day is much thinner than it was in the ice age , and the majority of it (85%) is found in Antarctica . About 11% of the remaining glacial ice is in Greenland, and the rest is scattered in high altitudes throughout the world.

Moisture and cold temperatures are the two main factors for the formation of glacial ice. Glacial ice in Antarctica is the result of relatively small quantities of snow deposition and low loss of ice because of the cold climate . In the middle and low latitudes where the loss of ice, known as ablation, is higher, snowfall tends to be much higher and the glaciers are able overcome ablation by generating large amounts of ice. These types of systems tend to be more active than the glaciers in Antarctica, and the most active of these are often located at high altitudes and in the path of prevailing winds carrying marine moisture.

The topography of the earth has been shaped by glaciation. Hills have been reduced in height and valleys created or filled in the movement of glacial ice. Moraine is a French term used to describe the ridges and earthen dikes formed near the edges of regional glaciers. Ground moraine is material that accumulates beneath a glacier and has low-relief characteristics, and end moraine is material that builds up along the extremities of a glacier in a ridge-like appearance.

In England, early researchers found stones that were not common to the local ground rock and decided they must have "drifted" there, carried by icebergs on water. Though geology has changed since that time, the term remains and all deposits made by glacial ice are usually identified as drift. These glacial deposits, also known as till, are highly varied in composition. They can be a fine grained deposit, or very coarse with rather large stones present, or a combination of both.

Rock and other soil-like debris are often crushed and ground into very small particles, and they are commonly found as sediment in waters flowing from a glacial mass. This material is called glacial flour, and it is carried downstream to form another kind of glacial deposit. During certain cold, dry periods of the year winds can pick up portions of this deposit and scatter it for miles. Many of the different soils in the American "Corn Belt" originated in this way, and they have become some of the more important agricultural soils in the world.

[Royce Lambert ]