STEVE EMBER: Other scientists have explained how the sand hills formed. They say a huge thick river of ice helped create the Indiana dunes. Thousands of years ago this glacier moved over what is now central Indiana. As the glacier moved, heavy ice crushed rocks into very small pieces. Over time, part of the glacier became a body of water called Lake Chicago, an early version of Lake Michigan. The melting glacier dropped the sand it had created around the lake. The sands of the present-day Lake Michigan are always moving. The winds and waves of the lake carry sand to the surrounding land.
Strong winds lift the sand and drop it on the land below. This process starts building new dunes.
FAITH LAPIDUS: Over time, plant life develops on these sand hills. For example, the cottonwood tree is usually first to grow on a new dune. Then the winds dig a hole in the sand. The winds use loose sand from the hole to create a large dune that moves. Such a dune can damage or destroy anything in its way. But cottonwood trees can help. The trees grow roots along their trunks as sand buries them. The cottonwood roots help hold the dune in place.
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STEVE EMBER: A dune called Mount Baldy guards the northern end of the Indiana Dunes National Lakeshore. Beautiful trees encircle its lower parts. Thousands of people climb the thirty-eight meters to the top of Mount Baldy each year. But getting there can be difficult. Climbers discover that their footsteps up the tall hill of sand often cause them to fall back again.
最新
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25
2013-11-25