Grid Reference SO 765389
Earth Science Conservation Classification EO (Inland outcrops)
This is a crag to the south east of Hangman’s Hill, reached by a path which branches off the Worcestershire Way Link footpath.
The crag is natural rocky outcrop on the hillside. It is made up of lavas and tuffs of the Precambrian Warren House Formation. The outcrop is a little rocky crag separated from Hangman’s Hill to the north by a deep cleft, the line of which is followed by the former Worcestershire Way Link footpath. On the north side the crag forms a rocky cliff face hidden under the trees and undergrowth. From this site there is a good view of the whole are of Castlemorton, Hollybed and Coombegreen Commons below.
This is an exposure of Precambrian rocks of the Warren House Formation.
They have been interpreted as deposited from volcanoes erupting in an island arc or marginal basin environment. Nearby similar rocks on Hangman’s Hill have been dated as 566 million years in age.
The site shows a wide variety of rock types and its geological history is obviously very complicated. There is more than one type of lava and several deposits of volcanic ash deposits (tuff). Many of the rock outcrops have a very rough surface showing the fragmental nature of the tuff (ash deposits). Some of the clasts are rounded and others angular. The rocks have weathered to a uniform rusty brown colour because of the iron minerals they contain. On some of the fresher surfaces green secondary epidote crystals can be seen. Some of the lavas are seen to contain vesicles, which were originally gas bubbles in which minerals such as epidote and feldspar have since crystallised. Thin sections show that there are highly altered keratophyres with rhyolite veins. The keratophyres are intermediate in composition between the mafic basalts and felsic rhyolites and are rich in chlorite, haematite and epidote.
There is a sheared zone running almost north-south and dipping 80 degrees to the east with foliation in the rock dipping at about 10 degrees to the south showing the line of movement. Towards the south of the exposure is a zone of fragmentary rock showing irregular flow structure. This overlies a rock type, which weathers differently giving a more massive appearance.
These lavas and tuffs were deposited in layers which have since been folded and are dipping to the east. A slickensided surface can be seen which has since suffered a degree of deformation.