Erratics:
A glacial erratic is a piece of rock that differs from the size and type of rock native to the area in which it rests. Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. When a glacier moves forward, it picks up chunks of bedrock as it moves bring them with it as it moves forward. As the glacier melts and retreats, it drops these boulders. Some end up buried under the ground. As the ground freezes and thaws it can slowly bring these buried erratics to the surface.
In Illinois, erratics are often granitic rocks like those you would find today hundreds of miles away in Canada. Erratics can be found all around central Illinois. Most glacial erratics appear worn and rounded, and sometimes include beveled or faceted surfaces. During the course of their journey, the rocks were jostled against other erratics or scraped against the underlying bedrock, rounding off corners and planing smooth surfaces, eventually producing their characteristic appearance. On some exposed rock surfaces you can find evidence of this glacial scouring in the form of glacial striations (long parallel scratches and grooves that are produced at the bottom of temperate glaciers by rocks embedded in the ice scraping against the rock underlying the glacier).
Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics were once considered evidence of a massive flood approximately 10,000 years ago. In the 19th century, scientists determined erratics as evidence for the end of the last glacial maximum (ice age) 10,000 years ago, rather than a flood. Geologists have suggested that landslides or rock falls initially dropped the rocks on top of glacial ice. The glaciers continued to move, carrying the rocks with them. When the ice melted, the erratics were left in their present locations.
By determining what type of rock the erratic is, it can be deduced where it most likely originated. Granites are generally from Canada, purple quartzite hails from Wisconsin, while those containing copper most likely originated in the Upper Peninsula of Michigan.
The average mass of a sandstone boulder is approximately 150 pounds per cubic foot. Limestone boulders and granite boulders in most cases weigh more. They average about 175 pounds per cubic foot.
To Log This Earthcache:
- Email/message me the answers to the following questions within three days (72 hours) of your find log. DO NOT POST ANSWERS IN YOUR LOG OR ONLINE.
- Approximately how many different TYPES of rock can you see in it. You can brag in your log if you can name any of the different types.
- Describe the color, shape, and texture of the erratic.
- Based on the colors of the minerals in this rock, where do you think it originated?
- Estimate the mass (weight) of the erratic in cubic feet. You will have to work out the volume of the boulder in cubic feet first, by "measuring" it on site. Find the length, breadth and height and multiply these three numbers together.
- Continue down the trail to the bottom of the ravine and locate another larger erratic near the bed of the stream. Compare the two erratics, and determine whether they were once part of the same piece of rock. If so, can you think of a reason why the large rock broke into multiple pieces?
- OPTIONAL: Post a picture of yourself and GPS-enabled device with the erratic in the background. The picture is optional, but helps me verify your visit and is much appreciated.
If these requirements are not met, your log will be deleted.
Please respect all local laws and park district regulations. Meadow Valley Park is open daily from dawn to dusk unless otherwise posted on site; no night caching please. Remember to leave no trace, and respect the local ecosystem.
Enjoy!