THE STRATIGRAPHY AND STRUCTURE OF THE ORDOVICIAN ROCKS OF THE RHINNS OF GALLOWAY

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Abstract
Re-examination of the stratigraphy and structure of Ordovician rocks in the northern part of the Rhinns of Galloway has led to amplification and some reinterpretation of the concepts of succession and structure advocated by Poach and Horne (1899). A stratigraphical succession within the greywackes has been erected on the basis of palaeontological and lithological evidence, and with the aid of “way-up” criteria. Stratigraphical groups have been subdivided on petrological grounds. The intercalation of at least two bands of Glenkiln (black) shale in a relatively thin greywacke sequence has been detected as far north as Broadsea Bay, and it can be demonstrated that a few miles farther north these black shales are represented by at least ten bands of fissile siltstone with black graptolite-bearing films, separated by some 6000 feet of greywackes. Relatively rapid cross-strike variations in thickness and lithological character have been found in several formations, amounting in the Glenkiln rocks to a general southward attenuation and concomitant increase in the proportion of shale and siltstone in the sequence. Palaeontological evidence shows that all the greywackes are of Glenkiln age, with the exception of the uppermost beds, the Portpatrick Group, which is probably Hartfell. Structurally the area is believed to comprise four main units, namely, three major complex monoclinal structures of Caledonoid trend with vertical northern limb and tightly folded and otherwise complicated southern limb, and one complex synclinal structure. The units are separated by major faults, mainly thrusts but including the Southern Uplands (= Glen App) fault, which is here normal, throwing south. Minor faults, especially wrench-faults, are of considerable importance.

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