Thursday, May 2, 2013

Wave Cut Terraces in the San Juans

I was messing around with LiDAR images of the San Juans as I had a request for use of an image from a past blog post and I am working on a few projects in the San Juans where LiDAR might be of help.

The messing around part was looking at wave cut terraces from post glacial isostatic rebound (with some additional local explanation isostatic-rebound-on-northwest-blakely). I started looking at bedrock areas as well. Most of the bedrock upland areas above the level of local late ice age sea-level are free of glacial sediment. This features could be used as a sea-level indicator as well.

That said I did find some interesting wave cut terraces.

Blakely Island

The Blakely image has been posted before. But it also shows the elevated bedrock area that has very little glacial sediment and for that matter very little soil development. The lack of smoothing of the bedrock by the glacial ice suggests that the ice filled in the fractures and joints and the ice flow simply was incorporated for the most part above the rock surface and the bedrock was not eroded very much. This is a common pattern throughout the San Juan Islands.

Part of Lopez Island

The image of Lopez is interesting in that wave cut terraces are restricted to the steep slope on the eastern side of the image. The upland area shows what may be glacial drumlins, but they are not well developed and possibly some sort of water flow or currents have altered this area while it was below sea-level.

Cattle Point, San Juan Island

Cattle Point is located on the southeast end of San Juan Island. This area is within the San Juan National Historic Park. It is the place where Americans set up camp during the joint occupation of the San Juan Islands with Great Britain post The Pig War. The Cattle Point area is also a great place to visit for metamorphic geologists as there are some great bedrock exposures along the shore with intense thrust fault shear zones and much debate about thrust direction.

But the wave cut terraces on the hillside my be the best in the San Juans. One possible explanation is that this area is fairly well exposed to open water so the wave energy was likely much greater here than other sites. The LiDAR also shows the small dune field that covers a section of wave cut terraces. The pit like structures sure look like kettles - places where blocks of ice were stuck and melted leaving a pit in the landscape.

Aerial image of Cattle Point (USGS)

The dunes can be seen in the aerial image. The kettle idea would require a field trip to confirm. The terraces show up a bit as the soil and slope aspects are just enough to alter vegetation growth habit.

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