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Amnicon Falls State Park Geology

Have you ever heard of the Douglas Fault? No, it's not someone's mistake, but the site of earthquakes that occurred about a half billion years ago. Amnicon Falls State Park has long been known as the best place to observe and study this important geological event and the associated rock formations. As you look at the Upper Falls, a billion years of geological activity lies before you. Evidence of volcanic eruptions, the advance of great oceans, the formation of sandstone, earthquake movements' and glaciation can all be seen.

The dark basalt seen at the Upper Falls is the solidified remnants of lava that flowed across the entire region about a billion years ago. The Lower Falls flow over Lake Superior sandstone. It was formed from sand deposited by streams flowing into an ocean that covered much of Wisconsin millions of years ago. The sandstone formed in horizontal layers, which you can see on the riverbank. The sandstone also has vertical cracks or joints. When the river undermined the rock wall on the northeast side of the river, large pieces of sandstone separated from the parent mass along the smooth joint planes. The result was a smooth cliff which you see here.

About 500 million years ago, there was a tremendous fracturing and movement of the basalt bedrock. The crack, called the Douglas Fault, extends from east of Ashland, Wisconsin, to near the Minnesota Twin Cities. The bedrock south of the fault slowly moved upward and to the north at a 50- to 60-degree angle.

More recently, great glaciers moved across the area. They brought different kinds of rocks, such as granite gneiss from the Canadian Shield region.

The fast-flowing water from melting ice and snow carried sand and rocks as it swirled around, wearing potholes in the basalt. The potholes hold puddles of water during low-flow times.

To learn more about this fascinating story, pick up a free booklet about the geology of Amnicon Falls at the park office. The booklet will guide you to stops along the walking trails at the waterfalls and explain how the tremendous forces of nature have combined to produce this state park's outstanding scenery.

Last revised: Wednesday April 04 2012