Wow! I came across this picture in a calendar and it really spoke out to me. What has happened to the sandstone is just what I envisage when I'm gouging and smoothing. The jewel collection of smooth pebbles in the bottom of the hole will always be there as it is quite a deep gorge. As the tide fills the hole, the pebbles will dance around in time to the waves and continue to erode the walls revealing the strata and variegated colour of the rock.
One of my moquettes has a separate 'pebble' perched in an opening. It has given me the idea of perhaps making more of this. Kate Timson has made many pebbles out of various clay bodies and pigments - I shall ask her advice.