Monday, July 18, 2011

Conglomerate Erratic and Link to Northwest Geology Field Trips Write Up of a Huge Erratic

Last week while working along a shoreline I came across a conglomerate boulder on the shoreline of Dabob by off of Hood Canal. The boulder was transported by glacial ice from an outcrop that is at least approximately 80 miles away. The boulder is a glacial erratic. I have seen numerous conglomerate erratics, but more often than not they are metamorphosed conglomerate. that is they were cooked a bit and slightly recrystallized. Meta conglomerates will break across the cobbles and pebbles whereas non metamorphosed conglomerates break around the pebbles and cobbles.   

Conglomerate Erratic

Close up of the pebbles showing surface of boulder broke around pebbles

David Tucker has become the gate keeper of Washington State erratics. He just put up a post on a huge erratic at the edge of a neighborhood park in Lake Stevens the-lake-stevens-monster-largest-erratic-in-washington-largest-in-the-us?. His posts on erratics are well worth checking out. For a magma and strato volcano guy he has been behavior has become erratic. (Yuk, but I could not help it)

As simple of a geologic concept as erratics are they are still a great deal of fun to discover. And it is great fun to speculate where they came from particularly when they are many miles from the nearest bedrock outcrop or are very out of place compared to the nearby bedrock exposures such as the erratic pictured above. This particular erratic appears to be a block of Huntingdon Formation or Chuckanut Formation from Whatcom County. Typically most people only wonder about the big erratics. However, David points out that even the pebbles in glacial drift are erratics.

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