You’ve heard the term “old as the hills”? Well, the Black Hills are pretty old. Some of the rock is estimated to be more than 2 billion years old. The granite spires west of Keystone (including Mount Rushmore) are relatively young, a mere 1.8 billion years.
"The geology of the Black Hills is complex,” according to Wikipedia. An understatement.
But that could change at any second in Keystone. With our altitude and the nature of Black Hills weather patterns, we are blessed with the occasional Chinook wind in the middle of winter.
Chinooks can range from a gentle puff of warm air to a rush of heat. They always bring a significant change in temperature.
How significant? Spearfish, to the north of Keystone, has held the world record for temperature change for more than seven decades. On the morning of Jan. 22, 1943, the temperature in Spearfish rose from -4 degrees to +45 degrees in just two minutes. That's a 49-degree change. Drivers had to stop in the middle of the street because their windshields suddenly frosted over.