The Rock Elm Disturbance

As this is an EarthCache, there is NO container. To claim this as a find you will need to complete the tasks outlined below. Failure to complete the tasks may result in the deletion of your log without notice. The terrain is mostly a flat, well groomed trail until you reach the rock formations where the terrain gets a bit uneven. Be sure to grab a trail map and head for the "underlook." The park entrance is located at N44 41.645 W092 13.280. There is a $5 per car daily fee but seniors (65 and older) are free.

Impact-
While there is some debate, mounting evidence appears to show that this area was struck by a 600 to 700 foot meteorite. Fossil study revealed that this event likely took place during the Ordovician Period, roughly 430 million to 455 million years ago when what is now Wisconsin was located just north of the Equator. To give you an idea how long ago this was, Wisconsin’s Ice Age ended 10,000 years ago and the Super Continent “Pangaea” broke up 230 million years ago...at the time of the impact the first vertebrates (fish) were still emerging and plants were about to appear on land.

It has been estimated that the meteorite would have been traveling at approximately 67,500 miles per hour. At ground zero the explosive force of the impact could have been more than 1,000 megatons (or about the energy equivalent in 63,000 Hiroshima size bombs). This is a force strong enough to have lifted debris more than 1,650 feet into the air and send crushing shock waves through the rocks.

An impact of this size comes along about once every 100 million years. Worldwide, there are only about 200 such impact formations known, and only a few dozen in the United States. In Wisconsin, the Rock Elm Disturbance (impact crater) is the largest of three known sites. The other two are near Pepin, on the Mississippi River in west-central Wisconsin, and Glovers Bluff in the central part of the state.

Crater -
The impact dislodged rocks and created a hole four miles across and up to 1,000 yards deep. Today it is difficult to spot surface features without a guide as the crater has eroded over time and been filled by shale, dirt and sediment. Adding to the process of erosion and also likely minimizing the initial crater size was the fact that a shallow sea existed here at the time of the impact.

Evidence -
Since the 1930's differences in the geology of this area have been documented. It wasn't until many years later that detailed studies would begin. In the 1980's UW-River Falls Geology Professor Bill Cordua began to look more seriously at the area. He started writing about the formation in 1985, and although he suspected it was formed by a meteorite, he could not yet prove it.

The rock structures outside of the Rock Elm disturbance are typical for the area. Layers of flat-lying sedimentary rocks can be seen where the rocks are exposed to the surface. At the base of the sedimentary section at lake level is a layer of sandstone called the Jordan Formation. Over the Jordan Formation is a layer of 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 when a shallow sea covered the Midwest 480-500 million years ago.

At the Rock Elm disturbance, this area's normally stable geology abruptly changes. Some bedrock is suddenly gone, dropped down, folded or faulted. Sections of older bedrock consisting mostly of shale have been uplifted 100's of feet to the surface. Younger sedimentary rocks unlike any found elsewhere in the region were deposited in the bowl of the disturbance. This may be a result of the disturbance bowl being filled by new sediment from the shallow sea.

This image depicts a typical impact crater. Breccias are one sign that an impact event such as an asteroid or comet struck the Earth. Breccias can be seen at Rock Elm. Also seen above is the central uplift and crater rim.

In the center of the disturbance is an area of uplifted bedrock. This "central uplift" exposes greatly tilted beds of Mt. Simon sandstone, a formation usually found in flat layers 700 feet below the surface in this area. It is not uncommon for central uplifts to form in the middle of large meteor craters. When the Earth is pushed down so quickly some of it will have a tendency to spring back up thus forming the central uplift.

A drop of water acts in much the same way as a meteor strike creating a “central uplift”.

Fragments within the breccia include granite, other igneous rocks, metamorphic quartzite and amphibolite, as well as sedimentary chert and sandstone. Some sandstone fragments found here are enclosed by glassy material, which could have been formed by melting due to an impact from an extraterrestrial body such as a large meteorite. Microscopic studies by Dr. Bevan M. French, Department of Mineral Sciences, Smithsonian Institution revealed fine fracture patterns in quartz grains like those known from shock metamorphism, giving greater weight to the extraterrestrial impact theory. Similar anomalous areas of deformed and faulted early Ordovician dolomite are present near Glover Bluff.

The coordinates listed for this EarthCache will bring you to a "U" shaped rock formation. Along the way to this location you will pass the coordinates for requirement # 2 below. You may park on the mowed grassy area near the trail-head located at N44 41.528 W092 13.477. While permission has been granted to park here, if the area is wet or muddy please park in a designated lot instead.

LOGGING REQUIREMENTS:
(Your log may be deleted if you do not follow these logging requirements)

1. You must take and upload a photo of the rock formation found at ground zero; the photo must include your GPS. DO NOT post a picture of the sign as it gives away the answer to #2 below.

2. Identify and E-MAIL me the colorful name of the rock formation listed at this coordinate N44 41.557 W092 13.574.

3. At the primary listed coordinates for this EarthCache (N44 41.546 W092 13.581) there is a "U" shaped rock formation. Measure the angle on each side of the "U" and E-MAIL me what the greatest angle is on each side. For this task you may bring along a protractor or use the one below.

*Remember- Do not post the answers for any of the requirements in your online log; answers must be E-mailed to me.

Permission for this cache has been granted by Park Manager Scott Schoepp.

Special thanks to Zuma! for cache intel and to Dr. William S. Cordua, Professor of Geology/Mineralogy, University of Wisconsin - River Falls for his assistance with this EarthCache.