Monday, April 7, 2014

LiDAR: Wave Cut Terraces on Orcas Island

LiDAR images heavily featured in articles on the Oso/Hazel Landslide and is a very powerful tool for identifying large landslide features http://seattletimes.com_mudslidelidar. With  little ground work and experience in an area LiDAR images also greatly assist in geologic interpretation. I was doing some pre site visit work for a couple geology hazard assessments on Orcas Island and noted a couple of linear features in the LiDAR (along with a few other features).    

LiDAR southeast Orcas Island

The terraces on the lower right appear to be wave-cut terraces. Similar wave cut terraces have been noted in previous posts (wave-cut-terraces-in-san-juans and isostatic-rebound-on-northwest-blakely).

There are some faint linear features in the upper middle of the image as well. The area where these features are located is generally bedrock, but it turns out these features are not bedrock, but instead are underlain by loose glacial deposits. I interpreted the units to be ice wasting deposits as they were non compact suggesting very late stages of glacial recession. Possibly they mark an upper area of glacial margin at a time when the ice sheet was partially grounded on the upper slopes of the island and floating on sea water.

The north end of Orcas Island near the town of Eastsound is where the wave cut terraces are more distinctive. Enough so that if you know what your looking for in areas of cleared of trees you might be able to see them without the aid of LiDAR.  

Wave cut terraces around the Eastsound area
Note as well the deep-seated landslide on the west shore

The Lappen (2000) geology map that covers the area has these areas as either undifferentiated glacial deposits or glacial marine drift. The drift is thick here with one area of at least 60 feet to the east of Eastsound. Elsewhere, on the mainland, Lappen called out previously identified wave cut terrace areas as emergent drift in the Birch Bay, Point Whitehorn, and uplands near Lake Terrell in Whatcom County and part of the uplands above Padilla Bay in Skagit County. LiDAR images of the area were not available in 2000, and as far as I know the wave cut terraces on Orcas had not yet been identified in the literature. Although I should note that Bretz (yes, the same guy that figured out the ice age floods of eastern Washington) described the wave cut terraces on the south end of San Juan Island. I had identified wave terraces on Orcas without LiDAR after Lappen completed the map compilation; otherwise, Lappen would have included them on the map. But identification of the terraces was over a very limited area until the LiDAR images became available. 
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 range 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 as seen above and elsewhere in the San Juans. The highest terraces I have seen 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 continued to melt as the ice age ended 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 bouncy crust beneath us. In other parts of the world such as Hudson Bay and Scandinavia the rebound is continuing and readily measurable even over historic times with former coastal communities now well inland.

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