A profile across the well known Menlo Hole. You can see that the hole is a feature of a fault line - with what appears to be a boulder field on one side, and a limestone / mud wall on the other.

The river has been the final resting place of hundreds of boats and water borne objects over the centuries. This is a small debris field, with a larger object near the centre of the image on the port side. It appears to have scored the bottom on its way down, or had simply dragged along in the strong currents.