Asymmetrical folding In the Hawick Rocks of the Galloway Area, Southern Uplands: comment

Abstract

SIRSStringer and Treagus (1981) have commented on our paper on the over-all folding in the southern part of the Southern Uplands . We are particularly interested in their contesting two of our conclusions, a) that folding, as measured in terms of fold hinges per unit distance, is unequally developed, and b) that there are essentially flat-lying areas interspersed by zones of steeply dipping beds. Both sets of authors have mapped the shore-line on a similar scale although in 1959 we did not publish our 1:2,500 maps nor the detailed cross-sections. For the sake of comparison we enclose a section drawn up by ourselves on the same scale as that published by Stringer and Treagus (1981 fig. 4, p. 136 from [NX 5815 4941]), of the section from Knockbrex through Corseyard to Kirkandrews (Fig. 1).We regarded the rocks along this section of coast as belonging to one of our flat belts. By a flat belt we meant that although the area was strongly folded, a notional stratigraphic horizon changed very little traced from one end of the section to the other, as can be seen from Figure 1. At the time we supposed that the enveloping surface of the flat belt (we used the German term Faltenspiegel) was essentially horizontal. The recognition of such stratigraphically flat zones associated with zones where the stratigraphic horizon changed markedly led us to the concept of a monoclinal structure stepping down to the NW. We were searching for an over-all model to replace the Lapworthian anticlinorium-synclinorium. The monoclinal model seemed to fit the data better although 20 years ago we could not visualize the stages leading to the deformation of a pile of horizontal rocks into the present over-all structure; steep belts always seemed to be too wide. The currently popular, accretionary-prism model resolves our problem by emphasizing faulting and relegating folding to a secondary role. Nevertheless it remains important there should be a clear appreciation of the detailed structural pattern.Is folding equally distributed? A simple test of this is to plot the cumulative number of fold hinges against distance across strike (Fig. 2). Two points emerge. The number of fold hinges per km as judged from the slope of the curve is similar over most of the Hawick rocks but as can be seen from Stringer and Treagus's sections (e.g. 1981, fig. 4, p. 137) the number is much more variable on a scale of hundreds of metres. The number does tend to decrease towards the south-east and is significantly lower in the Wenlock rocks of Meikle Ross. One steep belt occurs in the Wenlock.Are there flat-belts? Here we have to concede that the construction of a simple Scott.