This is a fascinating subject which draws upon a multitude of situations, locations and sizes.
My first photo illustrates the power of a rock and the impact it can make. found it during the summer of 2009. It is part of an old sea wall and the force of the incoming and outgoing tides must have caused the bigger rock in the middle to go spinning around in the hole causing it to gradually get larger each day with rock and tide rotations. Another example of erosion among rocks are the sea caves found in St. Martins, New Brunswick. Every day the power of the mighty Fundy tides moving in and out continue to sculpt these interesting cave formations.
Ever wonder why Newfoundland is often referred to as "The Rock" The photo of the huge rock of the shoreline might give you a hint. Still in Newfoundland , the rocky shoreline there provides ample opportunity for nesting by the many seabirds located in that region.
Thus the picture , nesting Kittiwakes.
Lastly, but still along a coastline we were studying the many rocks one day found in a breakwater at Saint John New Brunswick , when suddenly we saw a handsome black and white cat among the rocks. I have named this last picture "Breakwater Cat"