Tuesday, November 2, 2010

An Erratic Revealed


I spotted this glacial erratic just starting to be revealed by erosion along the shore of Hood Canal. This erratic is a block of the British Columbia Coast Range Batholith, one of the largest granite-diorite magma areas in the world. Think of Yosemite and the granite cliffs there and you have a good picture of the geology of the BC Coast Range. The big melt that caused these magma bodies to form took place between 75 million and 100 million years ago.

The BC Coast Range is a high range with lots of deep winter snow and many glaciers and ice fields. Those ice fields expanded and flowed south into the Puget Sound area and the Strait of Juan de Fuca several times over the past couple of million years. The last glacial advance took place between approximately 20,000 and 14,000 years ago. The flowing ice carried blocks of granitic rock like this one to Hood Canal. Erratics are a common site along the shorelines where the fine silts, clays and sand are washed away leaving the boulders behind on the beach. I spent some time working in the BC Coast Range traversing the margins of plutons and batholiths and always like seeing rocks from that area scattered on Washington beaches. Almost like seeing old friends as they bring back memories of good geo adventures.

2 comments:

helena.heliotrope said...

Have you seen the giant eratic in Seattle? It's about as large around as a mini cooper, and taller than me. It's near Ballard or Wallingford or Fremont...

Dan McShane said...

David Tucker at Northwest Geology Field Trips did a write up on some eratic trips. But I am note sure if any of the Seattle eratics were in the mix. Although I have seen lots of eratics I still love the story they tell.