Glacial erratic rocks differ from the native rocks to the area in which they lie by size and type. They can range in size from pebbles, to the largest known, which is Big Rock in Alberta Canada. Big Rock is estimated at 18,200 tons and is 29 feet high, 130 feet long and 59 feet wide but perhaps the most famous erratic rock known to us is Plymouth Rock, the site in Massachusetts where the pilgrims landed in 1620. The rock that stands before you was deposited by a glacier centuries ago after being formed in Ontario Canada. It lies directly upon glacial till which, in turn lies on a bed of sandstone some 443-416 million years old.
This particular rock is a metamorphic rock, examples of these types of rock include marble, slate, gneiss and schist but this rock before you is gneiss (pronounced nice). It derives it's name from the Middle High German verb gneist which means; to spark, so called because the rock often glitters. Gneiss is a relatively common form of metamorphic rock and there are many variations of it but all of them share some common traits besides also being metamorphic. These characteristics are that there are lighter and darker layers of foliated material and a coarse grained texture. These layers of foliated material can vary from as thin as a sheet of paper to over a meter thick. The repetitive layering you can observe in this rock was caused by a strong compressive force literally squishing minerals together in a perpindicular fashion under great heat or pressure differences. As a result the atoms recombine to form whole new minerals and the grains that were contained within the igneous rock that it started as, become the foliation that you can see. The related web link on this page has a great animation of the process in action. Being as common as it is, gneiss is commonly used for buildings, gravestones and monuments, including the famous Washington monument in D.C.
To log this earthcache you must complete the following tasks:
1. Tell me what direction the foliation on this rock is pointing. Do you think it is in line with path the glacier that dropped it took when it receded?
2. Do you notice any differences in the thickness of foliations along the rock? Why do you think it occurs if so? Other than color, do you see or feel any differences in the light and dark layers?
3. Imagine you could cut the rock in half. What do you think it would look like on the inside?
4. OPTIONAL: Post a picture of you or your GPS with the rock.
Send the answers to these questions to me either through geocaching.com's messaging service or through e-mail. If answers are not submitted for a reasonable amount of time after logging your find, your log WILL be deleted per earthcache rules.
Special thanks to park caretaker Trevor Davey and the town of Sodus for permission to place this cache! It is listed as permit #3 for Beechwood.