These unusual geographical features may be glacial or limestone rock in origin.These two are among hundreds near Mt Nimrod Reserve & aroused my curiousity from their inclusion on a topo map.Sometimes described as an upside down mountain,they are self draining & have no creek draining them.Great shelter for stock on the gale NW day we viewed them.5 metres or so in depth.A glacial version may be found near Hakatere Corner,on the way to Lake Clearwater.This one was formed when a large block of ice was left behind as the glacier retreated.It eventually thawed leaving a 50 acre depression some 30m deep.Some ice-cube!