John Nettleship* notes

The stream certainly seems to have had a reputation for appearing and disappearing quite suddenly. The Whirly Holes were swallow holes, a well known feature of limestone country, and I guess these had a more or less permanent existence as ponds. The most frightening event seems to have been a sudden loss of water, accompanied by a severe whirlpool effect.

He also notes*:

By comparison with certain landmarks the upper pool (or hole) from the 1841 map, is 46 long and 29 metres wide across its upper part. The lower pool, comparatively from the 1775 map is the same length, somewhat narrower, but its upper and lower sections are more equal. If they were holes rather than shallow ponds, they are large and could be quite scary, even without any violent movement of water.