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THE GEOLOGY OF THE ROCK ELM DISTURBANCE
by William S. Cordua, Professor of Geology
University of Wisconsin - River Falls

Nugget Lake Park sits on the boundary of one of the most geologically interesting areas in western Wisconsin, the Rock Elm disturbance. The disturbance is a circular region approximately 4 miles across. It contains anomalous rocks and structures thought by some to be the filled-in scar of an ancient meteor impact. The southern boundary of the disturbance lies within the park, north of Nugget Lake.

The rocks exposed around Nugget Lake itself show the normal features of rocks in western Wisconsin. They are flat-lying sedimentary rocks. At lake level is an orangish sandstone called the Jordan Formation. It is overlain by a gray bluff-forming series of dolostone, limestone and sandstone called the Prairie du Chien Group. Both the Jordan Formation and Prairie du Chien Group were deposited in a shallow sea that covered most of the midwest 480-500 million years ago.

To the north, in the Rock Elm disturbance, this predictable geology abruptly changes. Some bedrock is suddenly gone or dropped down. Other bedrock formations are strongly folded or faulted. Older bedrock has been uplifted 100's of feet to the surface. Younger sedimentary rocks unlike any found elsewhere in the region were deposited in a circular fault-bounded basin.

As one moves north through the disturbance one first crosses a fault and encounters a belt of folded and faulted Prairie du Chien Group rocks. These are well exposed at Blue Rock within the park. Other outcrops are found further north along both Rock Elm Creek and Plum Creek within the park and along County Highway HH northwest of the park. These rocks in places have been tilted nearly vertical. This whole belt has been dropped down relative to its usual elevations in the region.

Along Hwy HH one crosses a major fault. This fault drops the Prairie du Chien Group rocks still more. At the surface one now sees shales and sandstones that are not found elsewhere in western Wisconsin. These rocks were deposited in a circular basin formed after the major folding and faulting stopped. The shales, gray thin-bedded clay-rich rocks, are well-exposed in outcrops and roadcuts stretching from the entrance of the park north passed the little town of Rock Elm. A white to buff colored sandstone was deposited on top of the shales. This sandstone caps many of the hills in the Rock Elm region.

In the center of the disturbance, about 1 1/2 miles north of the park, a low range of wooded hills marks an area of uplifted bedrock that punches through the basin fill of shale and sandstone. This central uplift exposes tilted beds of Mt. Simon sandstone, a formation usually in flat layers buried 700 feet below the surface in this area.

The north edge of the area, north of Rock Elm, is a fault zone. This fault is circular and completely encloses the disturbance. It is traceable back around to Nugget Lake Park, passing through the park north of the boat launching area. But don't worry! These faults aren't active and haven't moved in millions of years.

One idea for the formation of the disturbance is that it is an ancient meteor crater. The meteor impact caused the folding and faulting and formed a crater that later filled with sediment. Central uplifts can form in large meteor craters when the middle of craters, after being pushed down so far, slowly rebounds, in the same way that the dents in your pillow disappear after you get up in the morning.

A different idea for the disturbance is that it originates from an internal burst of pressurized gas or water, channeled along very old faults. The gas burst would cause the folding, faulting and uplift. It would also form an explosion crater which would later fill with sediment.

The Rock Elm region has been the site of scattered finds of gold and diamonds in the stream sediments. Minding companies have explored the disturbance as recently as 1988. The gold is present as very tiny particles called "flour gold". During gold placer mining in the 1880's at least 10 tiny diamonds were discovered. Most were very small, the largest being 2 kts. Although not gem quality, they were pale blue to yellow in color. So far no local bedrock source for these minerals has been found. Their connection, if any, to the odd bedrock geology remains, like so much else about the disturbance, a mystery.