C— GEOLOGY. 75 



probable that the crest of the anticline was lower in the west than near 

 the centre. 



Tracing these various folds eastward it is interesting to find that they 

 do not continue far in that direction. The North Hill axis cannot be 

 traced beyond the latitude of Keyusham, while the Farmborough dome 

 appears to break up eastward into two shallow anticlines with an inter- 

 vening syncline. The depression between the Farmborough anticline and 

 the Mendips is replaced by shallow undulations. 



The Mendip anticline itself is probably a compound fold consisting of 

 two domes with their axes not quite in alignment, while there is some 

 evidence of a shallow syncline on its northern flank. 



The deep Glastonbury syncline appears at that place to be devoid of 

 pitch, judging by the parallelism of the strike lines in its flanks ; eastward 

 it is difficult to trace and probably dies away in minor undulations. The 

 swing in the strike on its northern flank, near Blackford and Chapel 

 AUerton, probably indicates the beginning of a subsidiary anticline and 

 of another syncline, with its axis north of the Glastonbury syncline. It 

 is probably in this subsidiary basin that the remarkable outlier of Brent 

 Knoll occurs, which rises abruptly from the alluvial flats to a height of 

 over 400 feet. The base of the Lias in this outlier is probably far below 

 sea level. 



The continuation of the Glastonbury syncline westward is indicated by 

 a boring at Bason Bridge four miles south of Brent Knoll, where the base 

 of the Lias lies at a depth exceeding 487 feet below Ordnance Datum. ^^ 

 Between Glastonbury and Bason Bridge there is, therefore, a decided 

 pitch along the axis of the syncline. Li the Shapwick boring situated 

 between these places the base of the Lias was proved to be more than 235 

 feet below O.D.'^ 



Regarding this map as a whole, it is obvious that there has been con- 

 siderable post-Liassic folding. More interesting, however, is the type of 

 folding which it reveals — that of elongated anticlines and synclines the 

 axes of which are impersistent, and tend to replace one another en 

 echelon. The character of the folding suggests that it is to be referred 

 mainly to one episode of movement ; on a small scale it compares with 

 the structures of the Jura Mountains as described by Heim. 



There is a remarkable similarity both in type and in scale between the 

 folding in this area and that proved in the marls of the Middle Chalk by 

 King (op. cit. sup.). This similarity is sufficiently striking to prompt the 

 question whether the characteristic features of the post-Lias folding in 

 the Mendips may not be of the same age and origin as the folds of 

 presumably Miocene age in the chalk of Northern France. 



It has been shown by Trueman, Cox and others that there were differ- 

 ential movements during the deposition of the Lias, leading to increased 

 thickness in subsiding areas and diminished thickness or stratigraphical 

 and palseontological breaks in areas of uplift or non-subsidence. Among 

 the latter the Mendip region is one of the most important, particularly 

 in the Middle and Upper Lias. The intra-formational movements resulted, 

 however, in much broader folds than those revealed by the present form 



••'' Wells and Springs of Somerset, p. 57. Mem. Oeol. Survey, 1928. 

 " Ibid., p. 63. 



