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It’s freezing! Ugh the heat! Brrr… grab a coat! Where’s my swim suit?! This weather has been up and down for so long! It doesn’t just wreak havoc on your sinuses. It also is really good at destroying rocks! Freeze-thaw is one way rocks are physically eroded (as opposed to chemical or biological erosion). This occurs when water seeps into cracks of rocks and freezes. In some regions, the temperature is warm enough for water to be liquid, but then at night the temperature drops to below freezing. Frozen water, or ice, expands, creating pressure on the surrounding rock and eventually cracking the rock further. When the temperature increases, the ice turns back to liquid and moves further into the cracks and the process begins again. This can cause significant change in landscapes, including the creation of huge piles of rocks on the sides of mountains. These are called talus piles.

Model of freeze-thaw weathering
Example of a rock that has been cracked from freeze-thaw erosion.
Long-term erosion results in a talus pile, rocks that have fallen from above.

It is also a great way to find, and open, Mazon Creek fossils! Located in the Braidwood area of Illinois, this area was strip mined for coal. The overlying shale was deposited in giant mounds that can be seen while driving through the area. Within these mounds lay treasure troves of concretions. During Chicago’s long winters, the weather can change on a dime, in the 60’s one day and back to the 30’s a week later. This might drive us all crazy, but for the Mazon fossils, it provides a great opportunity to break up the mounds of shale and lead to the exposure of concentrations.

Here’s hoping that one day soon we will all be able to get out and get back down to collect for these rare, but amazing fossils!

In the meantime, if you have a bucket of Mazon concretions lying around, now’s a perfect time to try your hand at the freeze thaw method of opening them and seeing what’s inside! Send your photos to our educator for identification! 

To learn more (or visit the historic Pit 11) check out this flyer: How to Open Concretions & Map! And visit the IDNR’s website for more information!