Sedimentary rock with gradded pebble bed at bottom of sample, and lumpy bottom in sandstone bed at top of sample. (click here for more explanation.)
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Geologists had long been puzzled by the existence of sandstone beds that seemed to have been deposited in fairly deep ocean. How did the sand get so far out into the quiet water of the deep ocean? Experiments, direct observation of the ocean floor, and theoretical calculations show that the sand is carried in a dense slurry of turbid water that can flow like a stream on the ocean floor. These density flows form where some near-shore event, such as a submarine landslide, a storm, or an earthquake, stirs up sediment and water, which then rapidly descend the sloping ocean floor. As the density flow slows down, the coarser particles settle out first, so that the final sandstone bed is graded from coarsest at the bottom to finest at the top. When the density flow comes to rest, its sandy bed presses down into underlying shales unevenly, producing a lumpy bottom. Many sandstone beds in the Nooksack Formation are graded; many have lumpy bottoms. Geologists can use these features to tell which side of the a bed was originally up. Very useful, indeed, where rocks are highly folded.
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