Landslides and Rock Falls

Topanga’s geology has more than one way to make the earth move.

Be careful what you wish for, even if it’s the end of a five-year drought.

This landslide just north of Topanga Living Café on Topanga Canyon Boulevard is certainly a constant reminder of our interesting geology. The opening in the rock is the result of a type of landslide called a block slide. A roughly 160 ton slab of sandstone on a preexisting crack slid downhill about two feet when rain added weight to it and lubricated the interface. (See diagram) The opening in the rock is the result of a type of landslide called a block slide.  A roughly 160-ton slab of sandstone on a pre-existing crack slid downhill about two feet when rain added weight to it and lubricated the interface. The slide plane is tilted by about 50 degrees, but it was originally horizontal. Tectonic forces rotated the sandstone and raised it to 1000 feet above current sea level.

Geologically, the sandstone is part of the Upper Topanga Canyon Formation. It started out as sand deposited as marine sediments in the Pacific Ocean on the continental shelf, roughly ten million years ago during the middle Miocene epoch long before what we know as California even existed. Since that time overlying sediments compressed the sand into rock. About 5 million years ago  in the Pliocene epoch, tectonic forces began lifting California from the sea to form the Santa Monica Mountains and our state began to take shape.

Topanga doesn’t change much on human time scales but over millions of years a lot can happen. Ten million years from now, Topanga will have been twisted and mashed beyond all recognition.

By Dave Lynch

 

No Comments Yet

Leave a Reply

Your email address will not be published.