GLACIAL EVIDENCE: IOWA'S DES MOINES LOBE
Continental glaciation was one of the most significant geologic processes to affect the landscape of the state of Iowa. Most of the materials from these glacial deposits can be seen at the surface here at this location on the Des Moines Lobe in northwestern Story County. The glacial deposits that were left here by the last glacial episode are lying just below the surface. These "remnants" are known collectively as "glacial drift"
The arrival of these ice sheets to this region began over 2 million years ago and over the years advanced and retreated many times. The glaciations can be recorded in the deposits found below the surface and date back 12,000-14,000 years ago. This is when the final ice sheet "retreated" (or melted) leaving the landscape behind that you see here at this site.
Despite being extremely powerful shaping forces across the Iowa landscape, only the remains of the final glacial episode can be seen today on the surface across parts of north-central Iowa. This final glaciation region, known geologically as the "Des Moines Lobe," is the last part of iowa to be touched by these massive continental glaciers. The Des Moines Lobe advanced first through Minnesota and South Dakota during the Wisconsinan (most recent) glaciation, across north-central Iowa and finally coming to a stop at what is now the city of Des Moines. Much of the river systems across this part of the state, including the Raccoon and Des Moines, are a product of this glacial meltwater.
Though different parts of the Des Moines Lobe possess very different physical characteristics, one thing is clear - drift was deposited in numerous places across this region and can be proven in a variety of different ways. This area of knob-and-kettle topography is outlined by sharp ridges of hummocky terrain are places where the ice sheet remained stagnant for a prolonged period of time, allowing the glacial material - or drift - to pile up and create small hills (or knobs) and leave potholes (kettles) on the surface. Other features, such as the larger and higher ridges and deposits, formed in an area where glacial ice was especially mobile, where it left thick deposits of pebbly till, creating sharper ridges.
These ridges outline the maximum extent of the glacial advance and the positions of prolonged, non-moving masses of ice are responsible for creating the landscape that you are standing on right now. Well-defined features were left here once the final mass of ice retreated from the region.
Please go to the set of coordinates listed above. If you look to your NE at about 60 degrees and approximately 0.2 miles, you will notice that the surface is significantly higher than the surrounding farm field. Please send these questions to me via email and DO NOT post them on the cache page when you log this cache! After taking a good look at this feature, please answer these questions:
1) Based on what you have read about the Des Moines Lobe, what type of feature are you looking at here? What is the approximate height of this feature?
2) Why is this feature here? When was it most likely formed?
Using the map provided (see the photo), please answer the following questions:
3) There are THREE regions of glacial advance across the Des Moines Lobe. This location is near the confluence of TWO of those regions. What two regions are these? This is likely the terminus (end) of which advance?
4) Based on the map provided, what type of terrain are you likely standing on right now?