LiDAR of south Bolton
The Twin Rivers Formation makes up the steep bluff slope on the western most part of the LiDAR with the tilted alluvial and glacial deposits located along the area of the bluff just to the left, west of the the first valley.
I do have a theory as to the valley. At the valley the sediments abruptly become horizontal and at least in appearance appear to consist of sediment from the last glacial period. If so, the valley was there before the last glacial period and was partially filled by glacial deposits. I have not been up in the valley to test the theory - so theory only at this point.
Whereas the sediments west of the valley are strongly tilted east of the both valley systems the sediments are much less tilted to not tilted at all. There are some glacial related sediments between the two valleys shown on the LiDAR, but east of the sharp narrow valley is an outstanding exposure of older alluvial deposits.
The exposure of old alluvial sediments runs for approximately 1,000 feet along the shoreline and the bluff is on the order of 200 feet high. The steep bluff itself is relatively stable except for shallow surface failures. Significant reaches of this bluff have recently been stripped of vegetation from a low angle landslide at the top of the bluff that is sending material cascading over the cliff to the beach below. It is a big low angle landslide. I measured one section of failing slope above the shoreline cliff with an angle of 12 degrees.
The bluff consists of alluvial sediment with alternating gravel and cobble units and fine grained silts as well as peat units. All the material is very hard and compact and the peat units suggest that a least a significant portion of this alluvial sequence was deposited during a non glacial interval.
Silt overlain by gravel with an intervening carbon rich layer
Layer of peat - a mat of compacted organics
More peat
The cobbles and gravel are a wide mix of material. I noted lots of andesite cobbles which caused me to think of the unit as being deposited by the Great Puget River. That is a river that would have been derived from all the west of the Cascades rivers gathered into one mighty river. Nice idea, but there are a number of alternative explanations - reworked glacial sediments being one of them.
Lots of puzzles to work out and one exposure will not solve the puzzle, but it is a great exposure. Birdseye indicated the presence of a older glacial drift unit on this bluff, but I could not confirm its presence at least in this section.
My interpreting and arm waving about this bluff exposure was secondary for my main purpose of being there. I was checking out the bluff because of the huge landslide that has been slipping over the past few years and expanded greatly this past winter. Another story.
Large scale landslides in the woods are hard to photograph
Dan,
ReplyDeleteThanks for the virtual field trip! We dated some organics at the contact of the sedimentary rocks and the Quaternary deposits. The date is infinite, suggesting that the nonglacial deposits are at least as old as Olympia. The tephra Birdseye noted (to the east,at the top of the Lindsay's Beach section), seems to be as old as Whidbey. Unfortunately the correlation of the tephra is between 100 to 200ka.
As for the structure...The bedrock drops to the east and the Quaternary deposits are deformed...we think there's a major structure passing through the area...Blakely's Dabob Bay fault zone?
Thanks, T. Contreras