Nested Rock

I happened to read the upcoming Challenges shortly before we went on a fall trip to the Olympic Peninsula.  While I was poking around the streambed of the Elwha river below where the upper dam used to be, I ran across this set of rocks and immediately thought of the challenge.  This kind of formation is caused by an eddy that makes the smaller rock vibrate or rotate on the larger and wear it away.  It also happens on much larger scales and you can find large kettles, particularly downstream of waterfalls.

5D3, 28-70 f2.8 L @ 70mm, hand held, f/8, 1/100, iso 1600, aperture priority, pattern metering, -1/3 EV.  Processed in LR5 & CS6.  Cropped mostly from the left with just a touch vertically.  Taken October 15th at 11:04 am.

This should be an interesting challenge, there are so many, many ways that water creates shapes.