Prairie
The term prairie is an ecological term used to describe a geologic plain covered by mostly grass. Prairies have been subdivided into smaller, more specific categories by the type of vegetation they support. Short grass and long grass prairies historically covered most of the central portion of the United States. However, the grasses have been replaced by urbanization and agriculture, but the plain still exists.
The Great Plains of the United States support one of the most famous prairies in the world. As with all prairies, the area is supported underneath by a firm bedrock. In this case, the bedrock is composed of limestone deposited by a relatively continuous series of ancient seas that advanced and retreated across America and Canada for millions of years. Dolomite, containing high levels of magnesium, is the primary building block of the bedforms.
Overlying the bedrock are massive and extensive fossil coral reefs. These ancient reefs are quite impressive. They began to form in the warm shallow seas after the Silurian about 400 million years ago. Growth was intermittent as the seas transgressed (grew) and regressed (receded) in a cyclic pattern.
During the last 1.9 million years, the entire upper North American continent was covered with ice. The ice sheets grew and shrank according to global climate fluctuations. The grinding of the massive ice sheets produced a fine sediment called glacial till. The meltwaters of the glaciers moved the till away from the sheets and out onto the dolomitic plain in a process known as glacial drift. The drift formed many distinctive structures including eskers, moraines, and kettles. The sequences of advancing ice are recorded in the layers of the sediments. What was once considered a distinct pattern of a few ice advances is now understood to be a complicated chronology of at least 29 different episodes. The last identifiable age of ice deposition is called the Wisconsin glaciation. This event stripped much of the prairie of high physiographic features while depositing the characteristic soils of the current prairie.
The soils that are high in carbonates are not very rich for growing vegetation. Trees are exposed to extremes of temperatures and varying precipitation. They do not fare as well as the hardier grasses. Consequently, the evolution of grasses has been intimately tied with the development of the prairie. The prairies have been threatened by the introduction of foreign grass types. Some areas of the prairie have been set aside as national grasslands and non-native grasses are sought out and removed. The majesty of the prairie still exists in these historic places.
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