Monday, November 16, 2015

Sheared cobbles

A few years ago, one of the graduate students in my geology department gave me some very interesting cobbles that have been sheared (fractured) and then the pieces were cemented together (thus they do not fall apart). All of this took place by natural processes. The result is that it looks like they have been sliced up, with the pieces shifted relative to one another.

The cobbles occur in the Upper Cretaceous Trabuco Formation in the northern Santa Monica Mountains, just south of the San Fernando Valley of southern California. This formation consists of cobble to boulder conglomerates with intervening coarse sandstones, all of which were deposited in an ancient alluvial fan adjacent to the ocean (i.e. a fan delta environment) approximately 90 m.y. ago. The locale has a lot of small faults and fractures. These sheared cobbles are silent witnesses to the strong deformation that has occurred to these rocks.

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