Large, cavernous sinkholes can form within the north coast’s marine terraces over time as ocean waves penetrate fractures and fissures in the bluff face bedrock, eroding channels and carving out sea caves, which then lengthen and broaden to the point of structural failure, causing the bedrock roof above (and the ground surface of the marine terrace), to collapse in on itself.
These alarming and spectacular craters can be found up the terrace several hundred meters inland from the shoreline, with the sight and/or sounds of surging ocean waters lapping in and out of the bottom of the immense pit through a small tunneled opening in the grotto wall.
Cretaceous bedrock (greywacke-sandstones-conglomerate) of the Franciscan Complex of the Coastal Belt, overlain by Pleistocene marine terrace deposits, forms the geologic building blocks of the North Coast’s marine terraces. Common coastal erosional features such as caverns, grottos, blow-holes, tunnels, arches, and sinkholes honeycomb this rocky coastline.
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