Friday, August 30, 2013

Geo 365: August 30, Day 242: Just Another Dike in The Wall

So here's the answer: "The Wall" is the product of a substantial dike, almost certainly of Oligocene age, intruding and baking surrounding sediments. The central Coast Range had a number of these intrusives during that time, likely associated with re-establishing the subduction zone following the accretion of Siletzia, the Coast Range block. Texturally and compositionally, this is a basalt, though it is intrusive, not extrusive. Basalt is much more useful as a road metal than the much more abundant and quickly weathering Tyee sandstones, so this restricted outcrop of basaltic rock had some value as a gravel quarry. Operations focused on the dike itself, and took a small amount of the surrounding contact metamorphosed Tyee sandstone, which, as a hornfels, is also tougher than the unaltered parent material.

"The Wall" is simply the remnant of that quarry: tougher, linear, tall, and quite sheer, after the dike was removed for gravel. The previous three days' photos have all shown contacts between the now absent dike and surrounding Tyee Formation, and yesterday's photo shows a couple of subsidiary dikelets squeezed into joints around the main intrusive (The areas highlighted between the pair of lines and under the ellipse.)

An acquaintance of my friend Bob contracted to put in a road a bit upstream of this spot, and when he was told, Bob asked if he found the dike. The contractor responded, "You knew about that!? I wish we had!" They had to subcontract out for drilling and blasting this obstacle and ended up losing money on the deal. Geology, no matter how simple, mundane and trivial, has the darnedest way of making itself important.

Photo by Dana Hunter, unmodified. May 6, 2013. Location uncertain, but maybe here.

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