Lying on my back in the stream I took another picture of my travelling companion, this time without my flash, relying purely on the light from his helmet mounted LED’s. Just before the sump the tunnel is rounded and pheratic in nature but in silhouette the entry tunnel takes on the distinctive outline of a keyhole.
A passage in this “keyhole shape” forms where the water table has dropped and the rounded tube that evolved beneath the water table is cut by a trench-like bottom. The new passage shape develops because of the running water’s erosive properties. Its scouring and solution effects above the water table are exclusively concentrated upon the passage floor.
It is a well known fact that local water levels have fluctuated quite drastically over the last several thousand years. Some might legitimatly argue that it was not the water table that dropped but rather the land that rose upward (isostatic rebound).