On some Natural Pits on the Heaths of Dorsetshire
Authors: Fisher O. 
Published: October 2009

On Affpuddle Heath and Piddletown Heath, near Dorchester, at an elevation of rather less than 500 feet above the sea, the surface is pitted with circular or oval hollows, like inverted cones, having occasionally a double apex. Their number is very great; and only the largest are marked in the Map of the Ordnance Survey.

They usually vary from about 60 to 80 yards in circumference; but one measures 130 yards, and another, called “Culpepper's Dish,” is 290 yards round: in the former the sloping sides are 23 yards high; in the latter 47 yards. After observing that these pits could not have been formed by the washing away of the underlying sand-beds, the author proceeded to show that their formation seemed to be due to the subsidence of the material into “sand-pipes” in the subjacent chalk, owing to the percolation of rain-water containing carbonic acid, which dissolved the chalk; and Mr. Fisher referred to the explanation of this process given by Mr. Prestwich in a paper formerly read before the Society*.

It is evident, that wherever a sand-pipe has been formed there a pit
must
have appeared on the surface, though the bottom of the pit may have been many feet above the chalk-surface; as we see a conical depression formed in the surface of the sand in the upper bulb of an hour-gass as soon as it begins to run. The Heaths in question would, therefore, illustrate the normal condition of every surface covering sand-pipes; and, if no pits occur, the surface must have subsequently been levelled by some agency or another. As this process could have gone on only during a subaërial condition of the surface, and must have occupied a very long time, the author remarks, that the larger pits on the Heath referred to must have been formed by the sinking of the Eocene beds into enormous “sand-pipes” during an extended geological period, and that the area they occupy was dry land during all that time, and has been so ever since, or the pits would have been obliterated; and must therefore have formed islands or headlands in the sea, which last filled the adjacent valleys and gave them their present configuration. The denuded chalk-valley separating the two Heaths is in most parts destitute of gravel, and contains numerous pipes full of Eocene materials, proving that there was a period during which large pipes were formed anteriorly to the denudation; while the nearness of the pipes to each other shows that the great number of the pits is no argument against the cause assigned to them.

As the formation of these pits was subsequent to the outspread of the superficial gravel of these Heaths, and previous to the last depression and elevation of the land, their date would be perhaps near that of the great mammalian fauna. The author also explained his views of the method by which the subsidence of the materials gave rise to the peculiar shape of the pits (for, however deep the original depression, and of whatever shape, the sides would always eventually assume the angle at which the materials would stand); and he observed that somewhat similar depressions have been noticed in the neighbourhood in process of formation at the present day.