The geology of the eastern part of the Lesmahagow inlier

Abstract

The Silurian rocks of the Lesmahagow inlier crop out over an area of approximately 80 square miles in the Ayrshire-Lanarkshire borderland (Fig. 1). The region is one of gently rolling moorland reaching maximum elevation on Nutberry Hill (1712 feet O.D.). It is drained by a series of small streams disposed about two watersheds. The courses of some of these streams have been in part determined by the fault pattern, notable examples being the Greenock Water and the Pockmuir Burn.Approximately 60 square miles, comprising the eastern two thirds of the inlier, were mapped by the author between 1958 and 1961 on the Ordnance Survey 6" sheets NS 62, 63, 64, 72, 73 and 74. Geological Survey of Scotland sheets 22, 23, 14 and 15 cover the area.Structurally the area is a complexly faulted pericline separating the Carboniferous basins of Douglas, Lanark, Ayr and Muirkirk (see Macgregor and MacGregor, 1948, p.12 and 13). Following the discovery of well-preserved eurypteroids by Slimon (see Murchison, 1856), the area has been studied by many geologists. Between 1855 and 1899 a great deal of collecting was done and many papers were published describing the unusual fauna of the area. In 1899 Peach and Horne gave a complete account of the stratigraphy and structure of the area and summarised the earlier literature (see Peach and Horne, 1899, Chap. 24 and Appendix, p.717). These authors divided the succession into 11 parts and considered that it represented an unbroken sequence ranging in age from (?)Wenlock to Lower Old Red Sandstone. The general anticlinal form of the inlier was described and its succession compared with that of the adjacent Hagshaw Hills inlier. Since 1899 almost all the work done on the area has been palaeontological. This work has been principally concerned with the age of the rocks, their correlatives in Scotland and Norway, and the importance of their record to the problem of the Siluro- Devonian boundary. (See Denison, 1956; Heintz, 1939, 1957; King, 1934; Lamont, 1947, 1952, 1955; Macgregor and MacGregor, 1948; Pringle and Ross, 1930; Robertson, 1957; Stetson, 1927, 1928; St9Srmer, 1934; and Westoll, 1945, 1948, 1958.)The objects of this work were as follows: 1. To remap the inlier and to re- examine the structural and stratigraphic interpretations made by Peach and Horne (1899). 2. To study the detailed stratigraphy of the Lesmahagow inlier and to attempt to reconstruct the sedimentary history of the region with particular reference to the relationship between sedimentation and tectonics.The laboratory work was done in the Grant Institute of Geology, University of Edinburgh and the Geologisches Institut der Universitat, W{irzburg, Bavaria.