New Explanation of the Great Glen Fault

Abstract

THE powerful dislocation which intersects Scotland in a north-east and south-west direction along the line of the Great Glen has usually been interpreted by geologists as a normal fault with a vertical downthrow on its south-eastern side. In an important paper read before the Geological Society of London on February 8, however, Dr. W. Q. Kennedy brought forward new evidence which leads him to believe the fracture as a strike-slip or wrench fault with a true lateral displacement of approximately 65 miles. It belongs, therefore, to the same class as the San Andreas rift of California and, like the latter, presumably extends downward at least to the base of the earth's granitic shell. This conclusion is supported by several independent lines of evidence, (a) The Great Glen fault belongs to the same system as the Strath-conan, Ericht-Laidon and Loch Tay faults, all of which have proved lateral displacements of up to five miles. (6) It possesses characters unlike those of a normal fault but similar to other strike-slip dislocations (for example, singularly smooth, straight outcrop and abnormally wide shatter-belt), (c) It displaces the great north-south injection belt of the Moine Series for a distance of at least sixty miles. (d) It truncates the Strontian granite, the southern portion of which, according to the detailed structural evidence, is missing. The missing half, however, has now been identified in the Foyers granite, which outcrops on the other side of the fault some sixty-five miles to the north-east and is similarly truncated by the fracture. These two intrusions consist of identical rock-types and agree in all details of internal and external structure. This interpretation clarifies certain obscure features of Scottish geology and serves to explain the southern continuation of the Moine Thrust in Islay. The fault is still active but the main lateral movement appears to have been accomplished in Upper Devonian or Lower Old Red Sandstone times. The dynamical interpretation of the movement, moreover, indicates that the fault must have resulted from a regional compression which acted in a north and south direction. The fracture is, therefore, unrelated to any Caledonian pressure or tension, but formed as a result of early Hercynian pressure.