Illinois Terrace Geology
The Manito Prairie is located on a sand and gravel terrace above the Illinois River floodplain. The bulk of the terrace was formed during the post-glacial period of the Wisconsinan glaciation with some sand deposition occurring later.
Figure1 Cross section of Illinois River Flood Plain and Terrace
The basal or lower deposits of the Manito Terrace consist of a combination of outwash sands and gravels from the Bloomington Moraine located to the east, and sands and gravels from the Kankakee Torrent, which flowed along the path of the present Illinois River valley. These deposits are overlain by finer grained sediments consisting of windblown sands that form the sand dunes and the finer grained silts and clays that make up the loess deposits. The Manito Terrace may include deposits from the Shelbyville and Leroy moraines below those from the Bloomington Moraine and the Kankakee Torrent. Due to the presence of the well-sorted sands and gravels in this area, sand and gravel pits continue to this day to be an important industry for Tazewell County. In order to understand how the Manito Terrace came to be, it is necessary to understand the Kankakee Torrent.
Kankakee Torrent
But, Kankakee is also the name of a river. A river that begins in Indiana and flows westward into Illinois, it joins the Des Plaines River to form the Illinois River. The modern Illinois River follows the ancient valley of the Mississippi for a long stretch of its flow.
Approximately 15,500 years ago glaciers to the north and east of the Kankakee area were melting -- melting very rapidly! Lobes of ice were in areas now occupied by Lake Michigan, the State of Michigan, and eastern Indiana. Melt waters were pooled by moraines to the south, and huge lakes formed, but not for long. the moraines were breached, and the result was among the greatest floods of the Pleistocene Epoch. This flood had impact not only in the Kankakee area, but in areas far away as well, southern Illinois in fact! Floodwaters were directed westward by a moraine to the south and eventually those floodwaters 'discovered' the ancestral Mississippi valley (which had been abandoned) and continue their route to the south via this path. The flood is know as the Kankakee Torrent. There were subsequent episodes of flooding, but none so great at this.
Many miles to the southwest, the Kankakee Torrent had an effect too. This was in the Havana Lowland. This is a shallow basin-like area between Pekin and Beardstown. Today it is part of the Illinois River valley. It was in this area southeast of Peoria that the ancestral Mississippi met the Mahomet-Teays River from West Virginia. This lowland had been modified by Illinoisan Age glaciers prior to the Kankakee Torrent. The tributary valleys had been filled with glacial drift, but the Mississippi and Mahomet-Teays Rivers remained. When a Wisconsinan Age glacier deposited its terminal moraine to the northwest, the Mississippi was diverted to the western route it follows today. The Havana Lowland and the abandoned Mississippi/Teays valley would await the Kankakee Torrent.
The torrential floodwaters moved immense amounts of sediment and eroded chunks of bedrock along its path. It also formed terraces over much of the surface. Geologists call the Illinois River an ‘under fit’ stream because it occupies a valley that is far too large for it to have been eroded by its own power. The former Illinois River Valley is clearly visible on remote sensed images as well as topographic maps.
The Manito Terrace:
As shown in Figure 1 the Manito Terrace is visible in a cross section the Illinois River Valley. Geologically, we know that sediment-laden floodwaters of the Kankakee Torrent and subsequent floods filled valleys with outwash incredibly quickly. In the glacial drift are found deposits of sand, gravel, and clay. As luck would have it, that "underutilized channel" mentioned earlier means that these ancient deposits are not swept away. Land elevation drops just 21 feet in the more than 200 miles from the head of the Illinois River to where the Illinois empties into the Mississippi. The river has so little energy that it cannot even carry away all the sediments dumped into it by its tributaries, much less sweep the valley of sediments deposited during its drastic glacial past. The sediments laid down by the Kankakee Torrent are still in place and have been supplemented by blowing sand and erosion from the surrounding moraines.
To log this Earthcache please complete the tasks as outlined below:
Drive to Terrace Location 1 below:
Terrace Location 1 N40 31.447 W089 46.239
1. What is the elevation at Terrace Location 1?
2. The higher elevation land behind you is designated for what land use. (May need to walk a short distance for the answer)
3. Picture of terrace (optional)
Drive to Terrace Location 2 N40 31.376 W089 44.552 Terrace Location 3 is N40 31 572 W089 44.555
4. What is the difference in elevation between Terrace Location 2 and Terrace Location 3?
5. Standing at Terrace Location 2 and looking towards Terrace Location 3, what manmade object is to the right of your current location on the flood plain?
6. Continue standing at Terrace Location 2. What type of business is located to your left? Why would you expect to find this type business here?
Hope you enjoyed learning some Illinois River Valley geologic history. The combination of glaciation and the Kankakee Torrent have made lasting changes to the Illinois River Valley. The terrace deposits of sand and gravel are of great economic benefit to communities all along the river. Another benefit is that the buried sand and gravel aid in the flow of aquifers and ground water which are also of great benefit. Please send your answers in a separate email to me. Logs without emailed answers are subject to deletion.
