Mount Rushmore National Memorial is carved from a giant hunk of granite laid down more than 2 billion years ago.
The Black Hills area of South Dakota and Wyoming is varied geologically and rich in mineral resources. It is one of the wealthiest gold-mining districts in the United States.
Types
The oldest rocks of the region formed in what is now South Dakota more than 2 billion years ago, in the Precambrian geological period. Mount Rushmore National Memorial is carved from a huge sheet of granite laid down in this era. The Black Hills region also contains rock formations from the Paleozoic, Mesozoic and Tertiary periods.
History
Many of today's mineral resources, including gold and silver, were formed in the Mesozoic era (248 million to 67 million years ago) when molten rock forced upwards through the eroded sedimentary rock above it.
Significance
The Black Hills district is renowned for its mineral resources. In addition to its deposits of gold and silver, the area contains petroleum, tin, iron ore, lead, copper, gypsum and mica.
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