Monroe Lake lies within the Norman Upland physiographic region. This region is typifi ed by steep slopes forming V-shaped valleys. The slopes range up to 50% in slope and are topped by narrow ridgetops. Most flat land is found along stream bottoms.
The rocks at this site belong to the Edwardsville Formation of the Borden Group. Their age is Middle Mississippian about 340-345 Ma. The Edwardsville Formation represents topset beds from one lobe of the Borden river delta that was prograding from the east into the Illinois Basin during this time.
The bedrock of the Norman Upland is largely siltstone, sandstone and shale with some limestone. The bedrock layers were deposited as delta and marine sediments. Bedrock is close to the surface and is exposed in streambeds and on some ridgetops. The fine-grained siltstones, shales and sandstones are cemented with clay and are highly impermeable. Limestone is visible at Allen’s Creek on the south shore of Monroe Lake. Fossil beds containing crinoids, brachiopods and bryozoans line the shore. These fossils represent marine life deposited when a shallow, warm-water ocean covered the region.
This site is abundantly fossiliferous, especially with crinoids. More than 80 species of crinoid have been recovered from Allens Creek. It was determined that communities were physically tiered similar to forests, with tall crinoids feeding high in the water and shorter crinoids feeding near the bottom. The fossil beds are filled with crinoids and corals, which are remnants of the shallow sea that once covered Indiana.
Unique to Allen's Creek
The vast majority of the rock that makes up the Edwardsville Formation is either sandstone or shale, derived from siliciclastic sediments that eroded off the newly-emerging Appalachian Mountains, traveled downstream via river, and were then deposited in the delta.
A notable exception to the dominance of sandstone and shale in the Edwardsville, though, is a large deposit of limestone at Allens Creek. This sediment material for this limestone did not come from the Appalachians. Instead, it originated in a bioherm.
A bioherm is an underwater habitat similar to a coral reef but dominated by non-coral organisms; in this case, numerous species of crinoids.
The rock at Allens Creek is exclusively limestone, rather than a mix of limestone with shale or sandstone, because the bioherm rested on top of an underwater bank.
Banks are raised areas that slope down in either direction – there is no direct link between the bank and the shore (there is an elevation dip between the shore and the top of the bank). This made it difficult for any siliciclastic material carried by the river system to reach the top of the bank.
Since the siliciclastic materials carried down by the river couldn’t “climb” upward onto the bank, the bank sediment came almost entirely from the carbonate-based animals that lived on the bank- hence, lots of carbonate limestone.
Limestone is a sedimentary rock, and along with shale, is one of the best preservers of fossils. Over time, sedimentary rock presses down around what were once living things to preserve the basic outline of their appearance and physical characteristics. Crack open a large piece of limestone or walk a beach covered with limestone rocks and you are almost guaranteed to find a fossil. If you don't find one, rest assured that they are there, lending the calcium carbonate from bones, exoskeletons and shells to the limestone itself. Various types of fossils commonly found in Limestone are as follows:
Crinoids
Crinoids are often called sea lilies, but they are animals, not plants. Complete crinoid fossils bear a strong resemblance to flowers. Their stem of small calcite disks are stacked like poker chips, and five feeding brachioles, or arms, extend from the top and resemble flower petals. The oldest crinoids are around 490 million years old and their relatives include sea cucumbers, sea urchins, starfishes and other echinoderms. After the animal died, its calcite disks would fall apart and scatter around the sea floor. Today, these incomplete crinoid parts are commonly found in lumps of limestone. Crinoid fossils appear as discs (when they are flat) or slightly rectangular (when they are fossilized on their side. Some of the discs may still be connected.
These fossilized segments of Crinoid stem used to be called St. Cuthbert’s beads. St. Cuthbert was said to have used them as rosary beads. In other places, the stem fragments are called screw stones because of the resemblance to the spiral of a screw. Some of the Crinoid segments have a five-sided shape and these have been called star stones. They were believed to be formed in the clouds and dropped during storms.
To take credit for this Earthcache you must do the following:
1. Find a crinoid fossil on the bank. What is its length?
2. Is the crinoid fossil similar in size to those around it?
3. Are there more types of fossils nearby? If so, what type? (if you don’t know the type, you can describe it)
4. Post a picture of yourself (face not required) or a personal item with a fossil you found here.
***** FOSSIL COLLECTING AT THIS SITE IS PROHIBITED. IT IS ILLEGAL TO COLLECT FOSSILS/ROCKS ON STATE PROPERTY. ANY LOGS INDICATING THIS WILL BE FORWARDED TO THE PROPER AUTHORITIES. *****