Pictured above is a common structure seen in the modern sediments of Stebbins Run. Geologists call it “pebble imbrication” or “shingle structure.” It is the tendency for loose pebbles, cobbles or boulders being washed downstream to tilt and overlap one another when they come to rest. Stream flow in this photo is from left to right. The rocks have to be elongated or tabular-shaped for this to happen. The thin-bedded shales and siltstones in Stebbins Gulch are ideal sources of slabby, rectangular rocks. When they come to rest on the streambed, their flat surfaces “dip” (tilt down) in an upstream direction. Pebble imbrication is occasionally preserved in ancient rocks, providing clues about the direction of flow in ancient watersheds.
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