Rock shaped by water

Today we processed core to a depth of 585.5 ft and logged boxes of rock to a depth of 325.9 ft.  One of the many interesting features we examined while logging is shown below (nickel for scale):

The piece of rock core on the left has an interior surface with a light coating of clay on it, indicative of low-temperature alteration of the rock via chemical interaction with water.  For comparison, the dark material at the top of this piece is a new, drilling-induced fracture surface that has not been altered.  Both of those features are common, but the strongly curved core interior is not.  It appears that over a long period of time, groundwater wore a smooth channel through this rock fracture.  Although this example is on a smaller scale, it is the same process by which rocks in a riverbed are rounded by the flow of water over them and rivers carve canyons down through solid rock.