The Stratigraphical Distribution of the Cornbrash: I. The South-Western Area

The detailed investigation of the stratigraphical distribution of the Cornbrash which has resulted in the present paper was commenced in 1921 by Mr. R. W. Segnit, who, however, left this country in the following year for Australia after completing little more than the study of the sequence in the immediate neighbourhood of Oxford. When, after the lapse of several years, there seemed to be no hope of his return, the present writers, one of whom (J. A. D.) had already shared much of the work, decided to continue the research into other areas, and together have completed the investigation of the formation from Oxford to the South Coast at Weymouth. That part of the outcrop now described runs through the counties of Oxfordshire, Wiltshire, Gloucestershire, Somerset, and Dorset. Since the publication in 1894 of H. B. Woodward's Geological Survey Memoir on the Jurassic Rocks of Britain, if we judge from the scanty references to the formation made by later writers, the Cornbrash has been regarded as of comparatively little stratigraphical interest, exhibiting only slight variation in character when traced along its outcrop, and admitting of no detailed subdivision. Although J. F. Blake, in his unfinished Monograph of the Fauna of the Cornbrash, the first part of which was published in 1905, gives no account of the zonal distribution, it has long been known that two main zones can be recognized: a lower zone characterized by Clydoniceras, and an upper by Ammonites of the Macrocephalites group. The critical faunas of these