In Wisconsin, United States
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Trilobites are the Wisconsin state fossil, and evidence of trilobites can be found along the Chippewa River, just downstream from the dam, where the Mt. Simon sandstone is exposed. Mt. Simon sandstone is the oldest of the Cambrian layers in Wisconsin.
510 million years ago, during early Cambrian times, what we now call Wisconsin was then located south of the equator. In fact, Eau Claire was about 10 degrees below the equator, and the land was under water, in a shallow sea bed. The diversity of life then was still exploding, with new forms appearing and disappearing, with the new forms themselves exerting new types of selective pressures to further drive evolutionary processes.
Most of the Mt Simon sandstone layer is buried quite deep, and this layer is only exposed in channels of rivers, and along some road cuts. Where the Mt Simon sandstone is exposed, fossils from that period may be found. One good place to find a good variety of fossils in Mt Simon Sandstone is along the Chippewa River where outcroppings of the 510 year old Mount Simon Sandstone have been exposed by river erosion. 510 million years ago, Eau Claire was beachfront property, with burgeoning life forms of the sea environment lying just offshore. The fossil record suggests an ancient environment comprising inter-fingering of shallow marine, fluvial and land areas.
If you look at the sediment layers of sandstone along the river here, you will notice that the layers have a variety of degrees of incline among their layers. Some of the layers are level, and other layers have inclines of 10 to 30 degrees. This is called crossbedding, and was caused by migrating underwater dunes. Since these dunes are formed in shallow water, we know that this area was on the coast of the ocean. And since animals were not yet living on land 510 million years ago, that confirms this area was under the sea.
No shelly animals or actual triobites are found along the Chippewa River here. However, there are a number of fossil trackways and body impressions, known as trace fossils, and many can be found near here. One example are the trails of the Climactichnites, that resemble tire tracks in the sandstone, evidence of an animal crawling on the bottom of the ocean. We found two near here, but neither were very obvious or large enough to be easily found, so I am not including finding them as part of the earthcache.
The coordinates above take you to two large rocks at the edge of the water. The sandstone is Mt. Simon sandstone, and here you will see hundreds of impressions of the bodies of trilobites. I visited this location with two geologists, and they called these impressions "Trilobite Butts."
However, the correct term for the trace fossil you will easily find here is "Rusophycus." When a trilobite is resting, partially buried in the mud of the sea floor, it leaves a bi-lobed impression called a Rusophycus. A diagram of how the trilobite leaves a trace fossil of the rusophycus can be found at: (visit link) That web site will show what you are looking for to fulfill the cache requirements for this cache.
Trilobites varied greatly in size. However, the trilobite butts (rusophycus) here are all pretty much the same size. Take a tape measure with you to the above coordinates, and measure the size of an individual trilobite butt here. Email me your measurements, and if correct within 1/4 of an inch, you can log the find.
Please tread carefully around these fossils, and others that you may find in the area. Several geology classes visit here each year from the university to seek these fossils and others along the river. Please do not remove any fossils or leave any marks.
Technical information for this location is credited to the book Roadside Geology of Wisconsin, by Robert Dott and John Attig. The trilobite butts at this location were identified by UW-Madison Geolgoy graduates Sarah Edwards and Jon . While visiting this area with Sarah and Jon and myself, WI_Robin got the hootie for being the first to find the trilobite butts here.
4/20 update: Added parking coordinates. To get to the proper place to park, go down Forest Street until you see the Park and Recs office, and turn into their maintenance lot. It looks like you are not supposed to be there, because of the garages, but once ya go 1/2 a block, you will see that it is fine, and that they have built a public access to the river just before the fence which keeps people off of the hydroelectric dam. Take the steps near the parking coordinates down to the river. There is no need to go beyond the fence.
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Last Updated: on 4/7/2013 4:29:58 PM Pacific Daylight Time (11:29 PM GMT)
Coordinates are in the WGS84 datum