Wednesday, October 10, 2012

Isostatic Rebound on Northwest Blakely Island

I was looking at LiDAR imagery of Blakely Island in the San Juans and noted very distinct former shoreline terraces cut into the slope on the northwest side of the island. 

Wave cut terraces in LiDAR, northwest Blakely Island

The terraces are cut into glacial marine drift. During the last glacial period northwest Washington was covered by a thick layer of glacial ice that advanced down into the low lands out of the coast ranges of British Columbia to the north. In the San Juans the ice was on the order of 5,000 feet thick. The mass of ice pushed the surface of the earth downward hundreds of feet. Towards the end of the ice age the ice began to retreat and sea levels began to rise. In the San Juan Islands the sea flooded the lower slopes of the islands before the glacial ice had fully retreated. A large area of mostly floating glacial ice covered the area. As the ice melted silts and clays as well as sand gravel and boulders would melt out of the ice and fall to the sea floor leaving a glacial unit called glacial marine drift.

But with the load of ice removed the land surface rebounded with local uplifts of hundreds of feet. Part of this uplift is recorded in the shoreline terraces cut into glacial drift on northwest Blakely Island. The upper terraces are approximately 330 feet above sea level. Hence, a minimum of 330 feet of uplift has taken place. However, sea levels continued to rise as ice around the world continues to melt and hence the total uplift has been even greater. Most of the rebound in the northern Puget Sound has been completed, we apparently have a very fluid mantle beneath us. In other parts of the world such as Hudson Bay and Scandinavia the rebound is continuing and readily measurable with former coastal communities now well inland.   

Wave cut terraces are present at numerous locations around the northern Puget Sound, but the best clearest terraces are in the San Juans with some that are readily identifiable even without LiDAR.  

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