Wednesday, March 07, 2012
Along the high bluffs north of Cama Beach, there is a deep box canyon cut back into the till. The upper end was pretty spooky when I first visited it in the 1990s, with vertical till cliffs on three sides, but a small window in the till at the base of the head wall offered a glimpse of the underlying fluvial sands. At the beach at the base of the ravine, there was a block of hardened debris that suggested some sort of big slide in the past.
There was a slide in the canyon last year - you can actually see the toe on the beach in the 2011 Google image linked at the bottom of this post. And then during the past couple of weeks, more material began sliding out of the canyon and onto the beach.
I estimated close to 1000 cubic yards of debris on the beach and an awful lot still in the ravine. Water flowing across the edges of the debris cone were depositing soft fans of sand, but the bulk of the slide was remarkably firm and dry and appeared to have arrived in an avalanche of fairly dry material. Most of the surface of the slide consisted of sandy material, but there were also large angular blocks (some 1-2 meters across) of till.
The source of the slide was massive failures in the high cliffs at the head of the canyon. Water was cascading from the forested surface (perhaps 140-150' above the beach?) down the face of the till - as I recall, the canyon coincides with a low point in the upland topography and may be drainage related. I couldn't get close enough to see if there was seepage from within the fluvial sediments themselves. Given the most recent failure last night, despite the lack of rain, and the current character of the steep cliffs at the head of the slope, I suspect additional large failures are likely.
The geology along these bluffs consists of till draped down over the seaward edge of the older flat-lying fluvial beds. In places the till comes all the way to to the beach, yet at the head of the slide, the till appears to be just 10-15' thick on top of the very thick sandy unit. You can sort of see this if you blow up the photo looking up the slope.
This is a fairly quick account - I'm sure I'll check back on this one -- and maybe the story will become a little clearer.
ADDENDUM: Here's a link to a cool video taken last week (not by me) - before the big stuff came down.
Landslide @ Cama