From Sandstone to Conglomerates: Sea Ranch’s Diverse Geology

(Click on any image to enlarge)

I recently spent an afternoon at a place my wife and I call “The Precipice” here on Sea Ranch.   It lies just north of Smuggler’s Cove and south of the Marine Preserve.  It lies within a geologic zone known as the Gualala Formation, as described in Ted Konigsmark’s book “Geologic Trips: Sea Ranch.”   Unlike the more northern parts of Sea Ranch’s coastal areas where sandstone is more prevalent, this area (and south to Bihler Point and Black Point) is composed of “Conglomerates.”    This formation is like sandstone, only it also contains large round cobbles about the size of oranges, and some quite a bit larger; even some boulder size rocks.   The cobbles are composed of many different types of rocks, as seen in the images I provided.

There is a reason that you find conglomerates forming points (e.g. Black Point, Bihler Point, “The Precipice”) in this region of Sea Ranch.  Conglomerates are very, very hard and resistant to wave and wind erosion.   The cobbles in the conglomerates were derived from the rocks that formed in the hills of the Gualala Basin some 80 million years ago.  These cobbles were carried from the hills to the sea in a dense stream of sandy sediment (called a turbidity current) where they accumulated in shallow water.  There are also sandstone layers within these deposits, and you can see some of these layers in connection with conglomerate layers in several of the images.

I suggest you take a look at Konigmark’s book if you want more information about the geology and geologic processes that formed this area.  Suffice it to say that it’s definitely worth the time to visit one of these areas here along our coast.  It’s an otherworldly experience to walk among the different formations here and try to understand how it all got here.   I hope I at least gave you a basic feel for the processes and the time involved in forming this unique coastal area.  But trust me, there is no substitution for getting out and hiking around these conglomerates to capture the full experience of such a special part of Sea Ranch and the Sonoma Coast.


Equipment: Nikon D810: Nikkor 14-24mm f2.8 lens; Monopod

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