Wurt Pit and Devil’s Punchbowl

Useful Information

Location:	South of Compton Martin and West Harptree on Old Bristol Road. (51.281495, -2.655870)
Open:	no restrictions. [2021]
Fee:	free. [2021]
Classification:	Doline Ponor Geotopes named after the Devil
Light:	n/a.
Dimension:
Guided tours:	self guided
Photography:	allowed
Accessibility:	no
Bibliography:
Address:
As far as we know this information was accurate when it was published (see years in brackets), but may have changed since then. Please check rates and details directly with the companies in question if you need more recent info.

History

Description

Wurt Pit and Devil’s Punchbowl are actually two different sites, the two largest dolines in the Mendips, and together they form a 0.2 ha geological Site of Special Scientific Interest (SSSI). At this place the karst is covered by insoluble rock, the solution and formation occurs only in the limestone layers below. At Devil’s Punchbowl it is covered by marls, at Wurt Pit by the Harptree Beds, an early Jurassic series of limestones and silica impregnated clays. The dolines form by collapses, when the caves below become too big to support the rocks above.

Wurt Pit is cup-shaped, 15 m deep and almost 100 m in diameter. The rim is sharply defined and the sides are steep and rocky, the reason are the silicified limestones and mudstones of the Jurassic Harptree Beds. The Mercia Mudstones and Dolomitic Conglomerate are below this layer, but they are not visible in the doline. They are believed to be at no great depth. The Carboniferous limestone was dissolved at depth, followed by the collapse of the cave chamber and subsidence of the relatively impermeable and insoluble cover rocks. It seems the cover rocks has fissures, so surface water was able to reach the cave below with its horizontal water flow. As it was not able to dissolve limestone in the overlying water it was still aggressive when it reached the limestone, mixture corrosion adding to the effect. As the doline is a result of collapse below and the depression is definitely not formed by solution, as the rock is insoluble, it is a collapse doline.

Devil's Punch-Bowl is 20 m deep and over 50 m in diameter. Here the Harptree Beds are mostly eroded and the walls show Mercia Mudstones, but there are some rotted siliceous rocks on the floor of the doline, which are remains of the Harptree Beds. Obviously the formation is qute similar, but the Mercia Mudstones are almost completely impermeable, so there was no or very little water from the surface. That’s easy to see because of the ephemeral lake nearby, the rain water forms such lakes in each depression and drains only very slowly underground. Also, because the material is softer, the rim is less rocky, the collapse was most likely quite unspectacular, more like a slow downflowing. Nevertheless, this doline is of the collapse doline type, as it is definitely not solutional. Just the type of collapse differs due to the different rocks.