The sink hole on Bronkam Hill has been in the news in the last few days. A hole was noticed in February resulting in the inland route of SW. Coast Path being diverted. However, the official route is now open as the area round the original small hole has collapsed and the sink hole is fenced off, so the area is now safe to visit. The sink holes are also called shake holes on the Ordnance Survey map of the area, but the correct technical term is a doline. These are very common in Dorset especially where sands and gravels (Bracklesham Group, Eocene age, around 50 million years old) rest on the Chalk. The depressions can sometimes be confused with bomb craters. They normally are seen with relatively gently sloping sides and covered in grass of heathland vegetation. (Picture 1 is of a sink hole on Bronkam Hill near the new example.) (Picture 2 is of the new sink hole). The new sink hole is 7-8 metres in diameter and 3.5 metres deep.
These features form over thousands or even millions of years. Rain which is slightly acid (pH 5.5) passes down through the sand and gravel soil and rock becoming even more acid. When the water reaches the underlying Chalk there is a chemical reaction. The acid water reacts with the calcium carbonate in the Chalk dissolving it, eventually creating a cavity underground. Over time this gets bigger and eventually the overlying sands and gravels collapse to form the sink hole. This is an example of geology in action and adds to the environmental interest making Dorset a special place to live.
Alan Holiday
