Corfe Castle is both an important site of history, as well as the geological processes that went into creating the icon hilltop that the castle is built upon. Corfe Castle was one of the first keeps to be built entirely of stone, and during its functional life it has seen battles, mysteries and plots. As well as a castle it also served as a treasury, military garrison, royal residence and family home. The keep was built in the early 12th century for King Henry I.
The position of the Corfe Castle was located, not only for its immediate defensible location, but also guards the entrance to the Isle of Purbeck through a gap in the Purbeck Hills from the North.
During the Civil War the Banks family, who were supporters of King Charles I held out during two sieges, until they were finally betrayed by one of their own soldiers. After this an Act of Parliament was passed to destroy the castle. Sappers dug deep holes, which they packed with gunpowder, that brought large sections of the wall down, resulted in large gaps and the unbelievable angles seen today. This is also the reason that huge sections of the walls are now resting half way down the mound, rather than on top in their original positions. These are known as 'tumble' and there are 74 pieces in total.
The castle was gifted to the National Trust in 1982 by Ralph Banks.
Topographic map of the area around Corfe Castle, with the modern town outlined by the black line.
The situation of the castle, perched upon the impressive steep sided hill, is iconic and entirely facilitated by the geology of the area. The castle is located on what would have been a continuation of the Purbeck Ridge, which can be seen extending both to the east and west of the castle. The distinctive gap created in this ridge was formed about a million years ago by the Byle Brook and Wicken Stream exploiting two natural weak points in the underlying geology. The resulting mound formed a natural motte, some some 55 metres high, that was exploited and the 21 metre high keep was located on top. The castle not only owes its impressive location to the Purbeck geology, but also for its very building material, which was sources from the ridge about two miles to the south. The economy of the area has long been dependant on its geology, with Purbeck Marble being an important exported building material, used in St Paul's Cathedral amongst others.
The geology of the Corfe Castle area, as shown in the map below, consists of a number of sedimentary rocks and clay units, including the Lower Bagshot Beds. These clay units are very susceptible to weathering and form the lower areas on the topographic map. The sedimentary rocks including Chalk, Lower Greensand, Portland Stone and Purbeck Limestone and because they are more resistant to weather than the clays they form the higher areas in the topographic map.
Chalk is a soft pure white calcareous form of limestone composed of the shells of millions of tiny sea creatures. When used as a building material Chalk is very susceptible to weathering, both physical and chemical weathering. Chalk is easily identified by its pure white colour and fine grained smooth powdery texture.
Greensand is a sandstone with visible rounded greenish grains. These grains are called glauconies and consist of a mixture of mixed-layer clay minerals, such as mica.
Portland Stone is a light grey limestone made up of tiny grains of sediment. Some varieties have shell inclusions, but all share the traits of being off white to light grey with, notably uniform in colour interspersed with darker specks or 'grains'.
Purbeck Limstone is a fossiliferous and part crystalline limestone that consists of highly packed shells in a lime mud matrix. It is harder and much more durable than most other types of limestones which makes it perfect as an external building material. It is easy to recognise because of its large shell content compared to other limestones.
Geological map of the area around Corfe Castle, with the modern town outlined by the black line.
To log this EarthCache please email or message me the answers to the following questions
At the posted coordinates you will find some rock faces
1) Describe the appearance and feel of the stone creating the rock face.
2) Using your observations and the descriptions above what type of sedimentary rock do you think makes up the Purbeck Ridge?
3) How tall are the exposed rock faces?
4) With reference to the geology and topography maps why is this ridge a prominent feature in the landscape today?
Go to stage 2 where you will see a number of wall tumbles
5) The walls of Corfe Castle are not made from the same rock as the Purbeck Ridge, instead they are made from the Purbeck Limestone ridge 2 miles to the south. Why was this chosen as a preferential building material?