As far as I know they are lined ponds used to collect dew and rainwarter, generaly found in elevated positions - I've seen them in the peak district and the shropshire hills.

What's to say it's only animals who drank for them? Or that their use was purely practical and not ritual as well?

Here are a couple of ponds:-

http://www.themodernantiquarian.com/image.php?image_id=11095

A close up of the left hand one

http://www.themodernantiquarian.com/image.php?image_id=11096

And a close up of a stone

http://www.themodernantiquarian.com/image.php?image_id=11097