OUCC Proceedings 11 (1983)

Helen Kay

Pozu Jorcada Blanca is developed in a grey, coarse-grained limestone containing occasional beds of thinly -laminated (approximately 1 mm thick) chert and occasional macrofossil-rich beds. The bedding of the limestone varies from thin to massive, with the thinly-bedded units tending to be more easily eroded, forming cutter-filled gullies on the surface and unstable vertical pitches underground. Although the structure of the area is relatively simple on the broad scale, the limestone dipping northwest to northeast at approximately 55-70° , on the small scale the structure is very complex. Many of the bedding features within the limestone show evidence of small-scale flexuring (less than 1 m) which could be syndepositional, and this is replicated on a larger scale by features of tens and hundreds of metres in size. A similar pattern of micro-faulting and small-scale faulting, with displacements of up to approximately 1 m, can also be recognised, but no evidence of large-scale faulting has been seen in the area.

Joints and tension features in the limestone are infilled by calcite, and occasionally by haematite. Vein calcite is also relatively common. Dolomitisation of the limestone is frequently seen, with particular beds usually having been preferentially replaced.

Unconsolidated superficial deposits of Quaternary age (presumably Late and Post-Glacial) are relatively uncommon in the area and consist mainly of angular boulders, cobbles and pebbles of limestone. These have been formed by both solution and gelifraction of the bedrock, the resultant deposits often forming chutes leading into and infilling shafts on the limestone surface. The process of infilling is presumably encouraged by the nivational processes operating in the shafts themselves.

In a number of locations, where the scree and clitter slopes have stabilised, and in small hollows and fissures on the limestone surface, finer-grained sediments have accumulated. Where these deposits have been stable for a number of years, a grey, sandy, carbonate-rich soil of generally low organic content has developed.

Given the recently-glaciated nature of the terrain, there is a surprising lack of morainic-type deposits in the area. The probable explanation of this is that the fine-grained component of such deposits has been reworked into the shafts and fissures which cover the limestone surface, and hence into the caves at depth.