Solid as a Rock?

We use the expression ‘solid as a rock’ to describe things and relationships we regard as unbreakable. A few years very unscientific observation of rocks - using my camera to photograph the same rocks on the same beaches at different times - has shown me that while they may indeed be extremely solid, rocks are neither unbreakable nor unchanging. The force of wind and waves can change the shape of a beach in a single storm - moving what yesterday seemed so large as to be unmovable to a new location overnight. Cliffs crumble, gradually or all at once; rain and salt water stain their surfaces with new, vibrant and unlikely colours, and the plants and animals of the intertidal zone - seaweed, limpets, barnacles - all leave their marks.