Most of the bedrock found in Illinois will be sandstone, limestone, or dolostone. But just inside of Lowden State Park you will find a piece of rock that is older than all the surface rocks in Illinois! On the north side of the entrance road to the park you will find a glacial erratic that was carried from its natural home in the Canadian Shield. It was transported there by one of the Pleistocene ice sheets.
So what type of rock is it? Unlike the sedimentary rocks that you will find in Illinois, this rock is actually igneous. When you take a look at the boulder you will see crystals in the white granite. You will also see how the lighter quartz and feldspar minerals are offset by flecks of the darker hornblende and biotite.
But look even closer and you will see another igneous rock type: one that looks like a dark-tone diabase. The geological name for this is a xenolith, which is a foreign rock body. It was embedded in the granite while the latter was still molten. The erratic boulder is Precambrian in age and the natural rocks in this area should be Cambrian and Ordovician. So it had to take a ride with the glaciers along with other rock formations that were created hundreds of millions years prior to the sedimentary strata that were deposited in Illinois.
To get credit for this earthcache, send answers to my account for the following questions:
1. Explain how this rock got to Illinois.
2. How big is the rock?
3. How big is the horizontal xenolith vein of quartz?
4. Compare the appearance of granite to the quartz
5. Post a picture of yourself (face not required) or a personal item at Lowden State Park.
Note: Geocache is placed on Department managed property with permission. It is the visitors responsibility to orient themselves with policies and rules pertaining to this Department managed site.
source: Geology Underfoot in Illinois