Is it a Chimney?

The two pictures below show the big chunky structure which stands to the East of the whitewashed toilet area. It's 5 feet square; what else could it be? We need to take the rubble off the top and look inside.

Old Map from 1880

Here's something we found on the internet http://www.old-maps.co.uk/maps.html

It shows the building with a recess on the South Eastern corner; presumably the wheel pit, though why they would show this as a feature if it had been redundant for 145 years I don't know.