The three diagrams here show a mountain landscape before a glacial advance, at the maximum extent of the glaciers, and after the glaciers have melted.
Glaciers don't cut brand new valleys. Instead, they flow down existing river valleys. In the landscape below, ice would begin accumulating in the shade at the tops of the highest valleys, and as it thickened, it would begin flowing downhill.
Since ice flows like a very thick fluid, it can't get into tight spots and erode like water can. Instead it simply planes off whatever is in the way, creating the U-shape so typical of glacial valleys. As the ice flows down from its source, it plucks rock away from the head of the valley, creating a bowl-shaped basin called a cirque. Some of the debris carried by the glacier is deposited along the sides to form a lateral moraine. When two glaciers join, their lateral moraines form a streak of debris in the middle of the glacier, called a medial moraine. Debris dumped at the end of the glacier is called a terminal moraine.
When the ice retreats, the former glacial valleys are easily identified by their U-shapes and cirques, and the margins of the glacier are marked by moraines. Medial moraines occur whenever two active glaciers join, but are rarely seen after the ice melts because melt water generally erodes them. Valleys that once joined the original river valley are now chopped off by the ice and end high above the valley floor. These hanging valleys account for many of the world's highest waterfalls.
Created 03 February 2008, Last Update 25 May 2011
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