Wetland Potholes EarthCache

There are several of these ponds in the woodland – the largest can be seen off of the trail that can be accessed from the parking lot at Lemont Road and 101^st Street – you can reach it by entering the trail system from the parking lot and proceeding north – after the trail makes a sharp 90 degree turn to the east it will shortly take another sharp 90 degree turn to the north and the pond will be on the east side of the trail.

The trail system in Waterfall Glen Forest Preserve makes a loop around the entire park. The preserve is geologically significant, featuring glacier-carved rock ridges, ravines and wetland potholes, which are not found anywhere else in the county! The 700-acre forest is a contiguous wooded acreage interspersed with dolomite beneath a prairie habitat, home to rare plants.

A pothole (kettle or kettle hole) is a shallow, sediment-filled body of water formed by retreating glaciers or draining floodwaters. The kettles are formed as a result of blocks of ice calving from glaciers and becoming submerged in the sediment on the outwash plain. Another source is the sudden drainage of an ice-dammed lake. When the block melts, the hole it leaves behind is a kettle. As the ice melts, ramparts can form around the edge of the kettle hole. The lakes that fill these holes are seldom more than 30 feet deep and eventually become filled with sediment.

Since the last glacial period ended some 10,000 years ago, most of the Midwestern United States (Illinois included) and Canada were covered at one point or another with a ice sheet. The ice sheet scraped large quantities of rock and soil away from the ground. As the ice blocks meled, they left behind depressions in the landscape and formed a majority of the lakes in the world.

The landscape is dotted with wetlands commonly called “potholes” or “sloughs” that were formed as Pleistocene Epoch glaciers receded around 12,000 years ago (Dyke and Prest, 1987). The wetlands formed where water accumulated in small depressions in a landscape that is underlain by low-permeability glacial till.

Wetlands can only develop in areas where the water can accumulate, so the ground must be able to hold the water.  Some rocks allow the water to penetrate rather easily; limestone absorbs the water and allows it to sink through the rock. Limestone can be dissolved by the water, especially if it is acidic.  This is how cracks and caves are formed. Sandstone also allows the water to penetrate between the sand grains.  This allows the water to drain from the surface.  Granite will not allow the water to drain. Granite is formed when volcanic molten rock cools.  When it crystallizes, the air and spaces are essentially sealed and do not allow the water to penetrate.  Similarly, shale and slate do not allow the water to penetrate because it was sealed by the volcanic activity and heat. Therefore, the type of rock plays a big part in determining where wetlands are developed.

Some rocks can allow water to collect if they collect particles of material that block pores in the rock.  Eroded material can be carried by flood waters that drain into a valley or low spot.  Clay is common in Illinois; and clay can be easily moved by water over long distances. Pieces of clay can settle into the pores of the rocks and block the water from penetrating, creating a wetland.

I am going to ask you to collect information as you walk along the trail.

1) Explore and describe the ground along the trail. Note how wet or dry each area is.

2) Explore and describe the vegetation as you walk along the trail.

3) Grab a stick, and see how far can you push it into the ground.

4) Look and explore at the soil in the water. Describe the texture and make up of this soil. Does it contain sand, gravel etc or is it just mud?

5) Explain in your own words as to how this pothole wetland formed.