December 16, 2012

Geology Word of the Week: G is for Glacial Erratic

Posted by Evelyn Mervine

Me, standing on top of a glacial erratic boulder in Nome, Alaska, Summer 2012.

def. Glacial Erratic:
A rock which has been transported and deposited by a glacier and which has a different lithology than the rock upon which it has been deposited. Often, erratic rocks have an angular shape because they were broken off of bedrock by glaciers and have not yet had time to be weathered and rounded by water, wind, and other erosional forces. Glacial erratics can range in size from very small pebbles to very large boulders, but usually it is the boulders which are noticed since these stand out in the landscape and are not easily transported away again.

Recently, I have been thinking a fair amount about glacial erratics and other glacial rocks and deposits, such as tills and moraines. That’s because I currently work for a marine gold exploration company that has a project offshore Nome, Alaska, where glaciers have transported gold to the coast along with erratics and other glacial sediments. If you walk along the beaches of Nome, you can spot quite a few glacial erratics, such as the one I’m standing upon in the above picture.

Another glacial erratic on the beach in Nome, Alaska, Summer 2012. Pen for scale.

Having grown up in New England, I’m no stranger to glacial erratics. In fact, back in September I wrote a little about my favorite glacial erratic, which sits on an island in front of my parents’ lakeside cabin in New Hampshire.

My favorite glacial erratic.