Geo 365: March 14, Day 73: Wave-Cut Notch and Structure

Here we've climbed up from the parking lot to just below the high stand of pluvial Fort Rock Lake, which filled this basin during the Pleistocene. A prominent notch can be seen on the flank of Fort Rock, just below the line of the horizon, but numerous other horizontal lineations suggest that lake levels varied quite a bit above and below the level of that shelf.

I didn't think to document the structure of this tuff ring as well as I could have (kicks self), but as I said, this was a stop that was much more rushed than I'd have liked- we scrambled up to this spot, stuck our noses up against the outcrop, snapped a few photos, and buzzed off again. However, if you enlarge the photo to full-size (right-click, "open link in new tab"), you can easily see that the layers of palagonite tuff are dipping toward the interior. However, if you look carefully at yesterday's photo (taken near the parking lot, near the base), it's more subtle, but especially looking along the top of the tuff ring, you should be able to see the layers are dipping toward the exterior. This allows some insight into what this feature may have looked like before wave erosion started tearing it up, and how it formed. This diagram, from USGS Circular #838, may help clarify. A somewhat more detailed description, and the source of that diagram, can be found in this road log at mile 36.1.

One last thing to notice: the lake sediments here hold abundant groundwater. So while annual precipitation is more appropriate for semi-arid grassland, and approaching desert-like conditions, water is being mined for agriculture, in particular, center pivot irrigation, which in satellite views dominates the flat lake floor. Such a plot is visible just below the horizon on the right.

Photo unmodified. August 20, 2011. FlashEarth Location.