Retrieved March 8, 2004, http://www.nps.gov/yell/publications/pdfs/laketrout2.pdf Yellowstone National Park Official Webpage. National Park Service. Retrieved April 20, 2004, http://www.nps.gov/yell/indes.htm
This underground reservoir covers 174,000 square miles. According to John Opie, author of Ogallala: Water for a Dry Land, the Ogallala was formed over the course of millions of years as the land flooded, dried out, and flooded again. As centuries passed, glaciers melted, carrying water, silt, and rocks from the Rockies down to the Great Plains to form the Ogalla. Dirt, clay, and rocks accumulated above it so that the waters of the Ogallala can now be reached at depths of 300 feet beneath the surface (29-35). Some people think that the Ogallala is a huge underground lake, but this idea is wrong.
Traveling north on an Indian trail, the first sign of the area’s cataclysmic past would have appeared out of place from the rolling hills typical of the Western Pennsylvanian landscape. Peering down into a valley over 400 feet deep, the mighty gorge was littered with enormous boulders, thus framing the Slippery Rock Creek. These relict boulders of rock types foreign to the area are known as “glacial erratics” and are indicative of the strength of the encroaching glacier. As defined by the National Snow and Ice Data Center, “Glacial erratics are stones and rocks that were transported by a glacier, and then left behind after the glacier melted. Erratics can be carried for hundreds of kilometers, and can range in size from pebbles to large boulders.
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