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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. "Erratics" take their name from the Latin word errare, and are carried by glacial ice, often over distances of hundreds of kilometres. Erratics can range in size from pebbles to large boulders such as Big Rock (16,500 tons) in Alberta, Canada.
Geologists identify erratics by studying the rocks surrounding the position of the erratic and the composition of the erratic itself. Erratics such as this one are significant because they are one of a series of indicators which mark the path of prehistoric glacier movement. Their lithographic origin can be traced to the parent bedrock, allowing for confirmation of the ice flow route.
Glacial erratics are formed by five different methods, four of which are caused by glacial ice erosion resulting from the movement of ice. Glaciers erode by multiple processes: abrasion/scouring, plucking, ice thrusting and glacially-induced spalling.
Plucking - Glaciers crack pieces of bedrock off in the process of plucking, producing the larger erratics.
Abrasion/scouring - In an abrasion process, debris in the basal ice scrapes along the bed, polishing and gouging the underlying rocks, similar to sandpaper on wood, producing smaller glacial till.
Ice thrusting - In ice thrusting, the glacier freezes to its bed, then as it surges forward, it moves large sheets of frozen sediment at the base along with the glacier.
Glacially-induced spalling - Glacially-induced spalling occurs when ice lens formation with the rocks below the glacier spall off layers of rock, providing smaller debris which is ground into the glacial basal material to become till.
Evidence supports another option for creation of erratics as well, rock avalanches onto the upper surface of the glacier (supraglacial). Rock avalanche–supraglacial transport often occurs when the glacier undercuts a rock face, which subsequently fails by avalanche onto the upper surface of the glacier.
This Glacial Erratic
In order to log this earthcache you will need to answer some questions, one of which you can answer immediately at the location, and one you might have to think about.
The answers to the following questions should be emailed to the cache owner when you log the cache, but should NOT be included in your online log:
1. Describe the size, shape and colouring of this glacial erratic.
2. Of the five methods listed above, which do you believe was the cause of this glacial erratic? Justify your answer.
3. Optionally, take a guess at how thick the ice was here and where this rock came from? (There used to be an information panel here that gave that information.)
4. Optionally, please take a photo near the published coordinates showing the glacial erratic and your GPS, but NOT the information panel. Upload this with your online log.
You can log this cache straight away after you have emailed your answers to the cache owner, no need to wait for confirmation. Please include the name of this earthcache in the email - you'd be surprised how many people forget. Also, if you want a reply, please include your email address. Any problems with your answers we'll be in touch.
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