Dr H. G. Reading queried the author's interpretation of the Moy Sandstone as entirely marine. The unimodal palaeocurrent directions pointing towards the basin centre, the feldspathic nature of the grits and the local derivation of the clasts would all seem to point towards a fluvial rather than a marine origin. Was there any positive evidence for a marine origin, such as polymodal palaeocur-rents, wave ripples, tidal currents, well sorted compositionally mature beach or bar sandstones ? The calcareous nature of the cement was no reason for postulating a marine origin because many fluvial sandstones have a calcite cement. Could this sequence of sandstones represent environmental changes from fluvial below into shallow marine above? If this was found to be so, it would suggest a southerly source area of much wider extent than the relatively small islands postulated by Dr Dixon.

In a written reply, the Author said, any impression conveyed that the Moy Sandstone had a history which was entirely marine was quite unintentional. Indeed the account of the history of sedimentation mentioned the accumulation of both fluvial and marginal marine detrital sediments during the Visèan transgression, although evidently it failed to give due emphasis to the initial fluvial aspects of the sequence. The isolated exposures along the headwaters of the River Moy show a pronounced change from dominantly cross-bedded, immature, coarse-grained, arkosic rocks in the lower part to planar bedded, near-mature, finer grained rocks in the upper part. This is in keeping with a sequence of environmental changes from

First Page Preview

First page PDF preview
You do not currently have access to this article.