Now to apply what you learned, here are a couple of logging tasks. Send the answers to me by email or the message center.
1. At GZ, take a elevation reading and let me know what rock group that elevation would fall under.
2. How does the rocks feel at this location?
3. Pour some water on the rock wall and observe what happens. Is it porous or non-porous? Is it permeable or non-permeable?
4. Using task #3, what type of rock is located here?
5. Post a picture (face not required) of yourself or a personal item.
With this EarthCache you will learn how this area changes from a river delta system to the actual man-made lake that you see in front of you today. Monroe Lake is actually the largest lake inside the state. So let's get started and learn why this lake is necessary.
The rock you see at this site was acutally not always in the form that you see today. In fact, this was all part of the Borden Delta of the newly emerging Applachian mountains during the Mississipian period. During the early part of this period, it was being shaped by a river delta system that flowed in a shallow sea.
The rocks were being washed into the river delta system and the larger pieces would fall out first and smaller rocks would go further. But the finer sediments of sand, silt and clay would be carried the farthest in the sytem before being deposited at the end of the delta where they are today. In this area, it was the finer silt depostions that formed the rocks that we see today.
On the west side of the lake you will see some of the limestone on top of the borden group. This was when the shallow sea deposited some of the animal remains that created the limestone.
So what kind of rocks do we find in the Borden group?
The bottom layer of this group is the New Providence shale. This is smooth mud that has harden over time into rocks. It erodes very easily into powder. You can rub your hand over the rocks and see the powder that is coming off easily. Since the river channels at this time were stable, the position of the deposits didn't change and led to a very consistent sediment layer.
The middle section of the Borden is the Spickert Knob. As the channels shifted, the depostion altered. It is not as uniform and along with the siltone and sandstones, you will also see some interbedded limestone from the seas.
The top layer of Borden rocks is the Edwardsville Formation. Sandstone and siltstone are dominate in this level because of the calmer waters that deposited the sediment. But this is the most important part of why we can have Lake Monroe in this location. Why you ask??
The Edwards Formation is non-permeable to water. It has siltstone and sandstone. So since water cannot penetrate the rock, it will hold the water. The Borden rocks are pourous, non-permeable Some of the rainweater is absorbed by the rocks, but most of the rain in the area will run off the suraces and goes into the Salt Creek watershed (Brown County and Southeast Monroe County). This water will accumulate in the stream valleys and moderate localized flooding will occur easily. Since the water will not penetrate the rocks, it will be sent further downstream and can overload the waterways to the south (Salt Creek, Clear Creek, White River, Wabash RIver, and Ohio River). So they decided to make a dam at this site to control that water.
So those same non-permeable rocks were used to create the basin for the lake. Since the rockbeds are tilted in this area, on the western edge of the lake some limestone is visible at the surface. At this point, the Borden Group changes to the Sanders Group and has limestone at the surface. Limestone is porous and has connected pore spaces so it will not hold water and can be a problem if the lake water reaches this point.
They needed a narrow point in the Salt Creek valley to place the dam. As mentioned before, limestone from the Sanders Group is close to the surface. Luckily only in the caps so the underlying rock will be Borden and will hold water. At the dam, 591 feet was rocks from the Borden Group. Then normal pool of the lake will be at 538 feet, giving a 53 foot buffer before the water would contact the limestone and create problems. Core samples were done at the dam to ensure that the Borden rocks would hold the water.
At the emergency spillway that you are now standing in, the elevation is lower than the dam. So water would completely bypass the dam and use this spillway during a major flood event and enter Salt Creek. The rocks that you are looking at are Edwardsville formation at the bottom half, and the top half is Rampcreek and Harrodsburg Limestone, which are both porous.
Now to apply what you learned, here are a couple of logging tasks. Send the answers to me by email or the message center.
1. At GZ, take a elevation reading and let me know what rock group that elevation would fall under.
2. How does the rocks feel at this location?
3. Pour some water on the rock wall and observe what happens. Is it porous or non-porous? Is it permeable or non-permeable?
4. Using task #3, what type of rock is located here?
5. Post a picture (face not required) of yourself or a personal item.
sources:
Jill Vance, Monroe Lake Interpretative naturalist
P. David Polly, Department of Geological Sciences, Indiana University