A Taste of the
Peak District geology, mud mounds, apron reefs …
Reef limestones present in the Peak District limestones are of two types, mud mounds (reef knolls) and apron reefs.
The Dove Dale reef limestone is light grey, fine grained and massively bedded. There are a few bedding planes visible and it is mainly unfossiliferous, although there is some crinoidal debris in places. Nowhere in Dove Dale is there any hint of the fossilised calcareous mud mounds called reef knolls (as seen in other limestones of the same age at different locations in the UK). At the end of Dove Dale, however, Thorpe Cloud and Bunster Hill are prominent examples of reef knolls. The mounds were held together by living organisms such as algae and bryozoa enabling the steep sides to develop. Normally mud mounds would have low-angled sides. Within these mounds there are localised pockets of very well preserved fossils but very little evidence of bedding planes. Evidence indicates that later limestones, covering these reef knolls was removed soon after deposition, prior to the Namurian shales and sandstones. The latter are thought to have been removed by the middle Triassic and that the current topography is much the same as during that period.
Chrome Hill and Parkhouse Hill, close to Earl Sterndale, are excellent further examples of the reef knolls and middle Triassic topography. During Namurian times, the shales that surround these two features were deposited in the valley. They were later re-excavated during the Ice Ages to leave us with the topography that we see today which is thought to be pretty much what they looked like during the Triassic.
In the Castleton region it is possible to see the transition from Dinantian times to Namurian times. During the latter, river delatas originating from the north laid down the shales and coarse sandstones that are known as Millstone Grit. Looking west from Castleton to the head of the Hope valley, one can see that the limestone forms a steep north-facing slope. Investigations show that it is very different to the limestone of Cave Dale. These are very fine grained, show little or no bedding with occasional pockets of well preserved fossils. These are therefore reef limestones and these particular ones have caused much controversy over their origin. The Castleton reefs do not show a mound-like form, causing problems with interpretation. It is now generally agreed that they represent an apron-reef, a marginal facies between a basin and a shelf, where the angle of the sea floor is changing. Evidence for this has come from the partial infilling of brachiopods found in the area of the Blue John cavern - these indicate a gently sloping sea floor at the time of deposition. At Castleton, the shelf was to the south with the basin to the north. The structure of the area can be subdivided into several parts, lagoonal shelf limestones, the back-reef, the algal-reef (stromatolitic algae), the fore-reef and the basin, which was then filled by the Edale Shales and covered by the Millstone Grit.
The lagoonal facies of Castleton is a crinoidal limestone with some corals and brachiopods. The back and algal reef rocks are oolitic in places and the only fossils are the calcareous algae. The fore reef has a wide variety of fossils and a variety of grain sizes and lithology.It can be seen exposed in Cow Low Nick, close to the south-east foot of Winnats Pass. Fossils in the reef limestones are noticeably different to the fossils of the shelf limestones. Pugnax is virtually absent in the latter but abundant in the reef whilst corals ae almost absent in the reef but present in places in the shelf limestone (mainly Lithostrotion and Arplexus)