Glacial Erratics: What Are They:
Glacial Erratics are pieces of debris, generally in the form of
rocks and boulders, left behind by glaciers. The debris can range
from small rocks to large boulders. Most larger pieces of debris
are ground into smaller pieces by the glacier containing them as
the glacier moves along its path. As the glacier melts any debris
riding atop, or within, the glacier is deposited where the melting
occured. Glacial Erratics are generally much different than the
native types of rock found at the erratics location. For instance
granite is an igneous rock yet there are no volcanoes located in
Illinois.
Granite erratics are very common in Illinois. The minerals
contained within are what gives the rock its primary color. The
minerals that are found in granite are primarily quartz,
plagioclase feldspars, potassium or K-feldspars, hornblende and
micas. Quartz is usually the last mineral to crystallize and fills
in the extra space between the other minerals. Quartz's hardness,
lack of chemical reactivity and near lack of cleavage give granite
a significant amount of its desirable durable properties. The
quartz will appear gray, but is actually colorless and is
reflecting and fusing the colors of the white and black minerals
surrounding it. The plagioclase feldspars are generally white with
a porcelaneous luster. The K-feldspars are generally the ones that
give granite its color variations from yellow to orange to pink or
blue. Dark K-feldspars can give granite its black varieties as
well.
Glacial Erratics: Where Did They Come From:
The Glacial Erratics found in Illinois typically came from Canada
and the States north of Illinois such as Wisconsin, and Michigan.
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.
When Did it Arrive?
According to Geologist Scott Brown with the ISGS (Illinois State
Geological Survey) the erratic that is this earthcache arrived
around 23,000 years ago.
Requirements:
- Judging by the colors of the minerals in this rock where do you
think it originated?
- What mineral gives this erratic its primary color?
- What glaciation period is this erratic from?
- Optional but encouraged: send a photo of your cache crew with
the erratic.
Gratz to Super Spartan 117 and SushiYummy on their FTF!