The picture to the left shows a person standing in the Roman passage to give it scale.  To the right is a  view looking up the Pot Shaft showing the almost square cross-section.

For comparison, we have reproduced a photograph on the right (supplied kindly by Martin Straßburger) that shows a Roman level in the Wallerfangen copper mines in Germany.  These were excavated in very similar Triassic sandstone.  The tunnel was driven away from the photographer.

Below is one of the shafts at Wallerfangen showing that the Romans were not too choosey about circular or square shafts.