Chapter 9 Sites with rocks of possible Lower Palaeozoic age

Introduction

J.N. Carney

This chapter features three GCR sites that include rock sequences previously thought to be Precambrian, but may be now referred to the Palaeozoic. The two Charnwood Forest sites have only recently been reclassified as probably Palaeozoic, and many users of this volume may still regard these rocks as part of the local Precambrian sequence. Describing these sites in a separate chapter, outwith the Precambrian GCR Block (Figure 1.5), emphasizes their distinction from the rest of the Charnian Supergroup and highlights a controversy that is still continuing. For the other site that has been included, at Thornton in Ribblesdale, a Palaeozoic age is strongly suggested even though the evidence is so far indirect, being based on a nearby borehole.

It should be noted that the Monian Supergroup of Anglesey is regarded by some to be Cambrian rather than Precambrian in age, as discussed in the introduction to Chapter 7, but the evidence again is indirect, being based on doubtful palaeontological identifications and correlations with rock sequences in south-east Ireland. One of the problems when considering the stratigraphical position of the Monian is that it occurs within an imbricated system of juxtaposed terranes (Figure 1.1), each representing a sliver of Precambrian crust that may be considerably removed from its original position. The Monian is therefore part of the highly complex structural fabric of Anglesey and for that reason it is better described, together with the adjacent rocks of definite Precambrian age, in Chapter 7.

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