To log this Earth Cache, please complete the following questions and send to the CO:
1. How many different glaciers affected this particular specimen? (hint: it's more than 1)
2. What characteristics can you identify that indicate the influence of multiple glaciers?
3. There is a plaque that notes another geologic force that acted on this specimen. What was it and what do you think means?
Description
If you spend much time wandering around in wilderness areas with exposed rock faces, you might notice long grooves in horizontal rock formations. Some of these grooves may have been caused by glaciers in a process called striation. Glacial Striation refers to scratches, gouges, and other linear scrapes in rock formations that were caused by glaciers.
Glacial striations on granite.
Whistler BC Canada (photo credit below)
As glaciers moved across the landscape, they picked up boulders, rock fragments, sand, and other materials that were capable of abrading the rock over which they traveled. The consequence of these abrasions can be seen in the forms of grooves or scratches.
The depth of the grooves depends on a number of factors, including:
- The depth (weight) of the ice (the heavier the ice, the more downward force is exerted)
- The shape and size of the materials (round and small materials are less effective abrasives than large fragments with non-rounded edges)
- The composition of the abrasive (it must be harder than the rock that is ultimately abraded.)
- The amount of rock fragments in the ice (too little= no abrasion, too much= changes in glacial movement behaviors).
- Presence of basal melt water:
- Some water between the ground and glacier will “rinse away” the abraded material (think of it as sawdust or flour).
- Too much water might allow the glacier to float over rocks that might otherwise be abraded.
- The speed of the glacier can also play a role.
A wide array of natural phenomena exert influences on geologic features, and in some cases these rocks have been "visited" by more than one glacier. It is possible to identify the presence of different glaciers by examining the depth and directionality of the grooves, among other characteristics.
Sources
- American Museum of Natural History
http://www.amnh.org/exhibitions/permanent-exhibitions/rose-center-for-earth-and-space/david-s.-and-ruth-l.-gottesman-hall-of-planet-earth/what-causes-climate-and-climate-change/how-climate-has-changed/how-climate-is-recorded/evidence-of-the-ice-ages/glacial-striations
- National Snow and Ice Data Center
https://nsidc.org/cryosphere/glaciers/gallery/grooves.html
- United States Geological Society
https://education.usgs.gov/lessons/schoolyard/glacialstriations.html
- Wikipedia
https://en.wikipedia.org/wiki/Glacial_striation
- Wikimedia Creative Commons (Photo Credit)
https://commons.wikimedia.org/wiki/Glacial_striation#/media/File:Glacial_striations.JPG
Congrats to SW00P, FTF