Crag Pit Nursery, Aldeburgh, Suffolk

[TM 458 580]

Highlights

Crag Pit Nursery is an important locality as it is the most northerly exposure of Coralline Crag available for study. It clearly shows the sedimentological and palaeontological features of the Aldeburgh Member of the Coralline Crag.

Introduction

The former Crag Pit Nursery was situated within this shallow pit which has existed since at least 1846 and may be the 'large quarry by the side of the road leading from Aldborough to Leiston' recorded by Charlesworth (1837b, p. 97). It was recorded in the Geological Survey Memoir for the area by Dalton and Whitaker (1886). The pit was also recorded by Harmer (1898) and was a popular collecting site for many years.

Description

At the present time this locality shows a vertical section of just over 2 m of Coralline Crag (Figure 10.26). Solution pipes penetrating downwards from the upper surface of the Crag were conspicuous as noted by Dalton and Whitaker (1886). The Crag shows horizontal or slightly dipping stratification typical of other localities in the Aldeburgh area.

The Crag sediment here has been leached of all aragonitic skeletal material but evidence of the aragonitic fauna is preserved as moulds of large bivalves and occasional internal moulds of the large gastropod Scaphella lamberti. The sediment at Crag Pit Nursery consists of over 85% carbonate.

The calcitic fauna is, in general, abundant and well-preserved. Most notable are valves of Aequipecten and Pecten, which often show a diverse and very well preserved and delicate encrusting epifauna of bryozoans and occasional serpulids on their concave surfaces. The occurrence of these and frequent clionid borings in the molluscan shells is evidence of a relatively slower rate of deposition in this offshore facies. Current winnowing of fine-grained sediment may have been important in maintaining a coarse, clean, carbonate sand which suited development of a diverse epifauna. Some fine sediment is present, however, in the form of small lenses only a few centimetres long. These scattered lenses may represent sediment trapped in burrows or even by ripple troughs. Other notable fauna include large, well-preserved colonies of the bryozoans Meandropora and Blumenbachium globosum (syn. Alveolaria semiovata). Whilst Meandropora is also found in localities in the south-western part of the outcrop, e.g. at Ramsholt Cliff, B. globosum is apparently only abundant in localities around Aldeburgh.

Interpretation and evaluation

This pit is the most northerly exposure of Coralline Crag on land. Offshore, the Coralline Crag is exposed on the seabed off Sizewell a few kilometres to the north-east (Lees, 1982; Balson, in press). The sediment and fauna exhibit many similarities to that at Aldeburgh Hall. The fauna is diverse and well preserved, occasionally with an abundant epifauna. By contrast the large cyclostome bryozoans, so common at Aldeburgh Hall, appear less common at this locality.

The high carbonate production and diverse, well-preserved encrusting epifauna are evidence of reduced terrigenous input and current winnowing. This locality is representative of the 'source area' for carbonate sand which was swept by tidal currents along a linear sand bank to the south-west.

Conclusions

Crag Pit Nursery is an important site in the Aldeburgh Member as part of a network of sites which illustrate the variations in fauna and facies in the Coralline Crag.

References