Cambrian, Bristol and Gloucester region

Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)

The Wye valley, looking northwards from Symond's Yat to the Old Red Sandstone ridge of Coppet Hill (A6262). (P206191)

The oldest rocks that crop out within the district belong to the Tremadoc Series, traditionally regarded in Britain as the youngest part of the Cambrian. These Tremadoc rocks occupy an inlier that extends northwards from the Tortworth area nearly as far as the River Severn at Tites Point (P2061910). They are very poorly exposed and knowledge of the succession has mainly been gleaned from temporary exposures. They were first described by Smith and Stubblefield in 1933 and subsequently by Curtis (1968), while more recent mapping by BGS has thrown additional light on their structure. The succession is predominantly of micaceous shale, grey when fresh, with a variable proportion of interbedded siltstone or very fine-grained sandstone layers, lenticles and beds. The total thickness of the exposed rocks cannot be established with any accuracy. Calculations based on average dips and width of outcrop give a thickness of some 2200 m. This figure takes no account of tectonic thickening by folding and faulting, but a comparable thickness of mainly Tremadocian shale overlying Upper Cambrian shales and Lower Cambrian sandstone or quartzite has been proved in the Cooles Farm Borehole at Minety, less than 40 km to the east. Seismic reflection evidence suggests the extension of these rocks at depth along the entire eastern margin of the district.

The succession at Tortworth, which in broad terms youngs southwards, is divided lithologically into the Breadstone Shales below and the Micklewood Beds above. The base of the Tremadocian is not seen but the top is overlain unconformably, with slight angular discordance, by Silurian rocks; the Ordovician is not represented. Marine faunas have been collected from both formations and these show similarities with the North Wales Tremadoc faunas rather than with those of the geographically nearer Welsh Borderland and Midlands areas. It has been suggested, on the basis of numerous minor sedimentary structures in the sandy units, which are particularly abundant in the Micklewood Beds, that the rocks were deposited in relatively shallow agitated water.

Breadstone Shales

These comprise about two-thirds of the inlier. The interbedded siltstone layers are rarely more than a few centimetres in thickness. Over much of the area the fauna indicates a lower Tremadoc age (Rhabdinopora [Dictyonema] flabelliformis Zone) and includes horny brachiopods, bellerophontoid gastropods, trilobites (Beltella, Niobella) and ostracod-like forms, as well as the zonal index fossil. A record of the graptolite Clonograptus sp. suggests that the overlying Clonograptus tennellus Zone is represented in the northern part of the outcrop.

Micklewood Beds

These occur over the southern one-third of the inlier and, although the boundary with the Breadstone Shales is not clearly defined, are usually distinguished from them by the more common occurrence of flaggy sandstone or siltstone beds. These beds are variable in thickness, but typically measure between 15 and 30 cm. The zonal age of the lowest 60 to 90 m is unknown, but the remainder of the succession is Upper Tremadoc in age. Some layers are crowded with fragments of the horny brachiopods Lingulella and Schmidtites, whilst the trilobites Angelina and Peltocare have also been recorded.