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Site 11 - Combe Down Oolite/Twinhoe Beds
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From site 10 walk back to the gate (do not go through) and go down the path parallel to the fence until you find a path going steeply
down through the wooded area, (GPS N 51o23.695' W 002o17.861') Continue down until you reach a made-up track, (GPS N 51o.23.740' W 002o17.878'). On the way down, the rocks that lie below the Bath Oolite are seen on the right hand side. Turn right along the made-up ride to reach a large exposure and large fallen blocks on the right hand side, (GPS N 51o23.765' W 002o17.801'). At this site there are two high rock faces separated by a scree and in front of the faces large blocks of rock that have broken off from the main rock mass and slipped or rolled away. It is possible that some stone was quarried in this area for local use resulting in the faces and crags seen today, but it is likely that some at least are the result of natural processes, chiefly slip down the hill on the underlying Fuller’s Earth Clay. The rock faces are reached by a short scramble up the slope. The right-hand face consists of the Combe Down Oolite, the oldest rock seen on the reserve. This rock is clearly different from the other oolites seen so far. It is darker in colour and contains a considerable amount of fossil debris and some small whole fossils. The rock is not suitable for working as a freestone here, but south of Bath at Come Down and Odd Down its nature changes and it has been and still is worked for building purposes. It is less porous than the Bath Oolite and therefore withstands the weather better. At the bottom of the face is a small platform of hard massive rock with, above it, a rock that is clearly layered and which has fallen away resulting in the face being undercut. If the layered rock is examined carefully at a clean edge, the layering can be seen easily. Small fossils are common on the faces of these layers and at some levels burrows are also present. On the left hand corner of the face cross bedding structures have weathered out. Cross the scree to the left hand face. This is again the Combe Down Oolite and the base is deeply undercut forming a cave. It seems probable that the cave was made by the removal of stones for local wall building but it may be a more advanced stage of the undercutting seen at the right hand face. Climb the scree at the right hand side of the face and locate a band of rock containing a large number of brachiopods, (see illustration). The rock face has been cleaned to show these and MUST NOT BE HAMMERED. Note that when a section through a brachiopod is seen, the inside of the shell is rimmed with calcite and the rest of the shell is empty, see illustration. What does this tell us about the brachiopod shell when it was buried initially? Why should there be so many brachiopods in one place? The band of brachiopods is about 1m below the top of the Combe Down Oolite. Above this are the Twinhoe Beds, the name being taken from the village of Twinhoe near which they could once be well seen in an old quarry. These beds consist of a wide variety of rock types from hard shelly limestones to soft clayey bands, the differing nature of the beds being reflected in the way they have weathered. Some beds are very fossiliferous, containing bivalves, brachiopods and small gastropods. The beds immediately above the Combe Down Oolite are quite distinctive in hand specimen, a fresh surface showing dark brown rims surrounding many of the ooid-like particles. The brown rims are the result of replacement of the outside of particles by an iron oxide and this type of rock is often called ironshot. A detailed examination of the rock (see illustration) shows that true ooids are quite rare, the ooid-like particles being composite grains of ooids and small shelly debris bound together by an algal growth, though the exact nature of this cannot be determined because the iron replacement has obliterated all detail. The fallen blocks appear to be of Combe Down Oolite and the very variable nature of this rock will be become evident if samples from various blocks are examined. The variable nature of the Combe Down Oolite reflects the complex changing environment in which it was formed. By analogy with environments where similar sediments are being deposited today, it is considered that they were formed in shallow water with strong tidal currents. It is unlikely that there were many indigenous organisms or active ooid formation; both of these having been swept into the area from outside. At times, stable conditions would have existed and the brachiopod bed indicates one of these periods when conditions were stable and large numbers of brachiopods became established. The fact that the shells of these are now rimmed inside with calcite and no sediment being inside the shell suggests that they were shut tight when they were buried and they may well have been buried in their life position. |
Site 11 (Mell Freeman)
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