Henry Bury — Escarpments and Transverse Rivers. 121 



which produced the present structure of the Thames Valley, there 

 was a fall of sea-level or a rise of the sea-bottom that brought the 

 beds within reach of marine erosion. As a result the sea steadily 

 ate its way far inland along the outcrops of the two masses of soft 

 material, the Eocene beds and the Gault, leaving four projecting 

 ridges, the two inner ones formed of Chalk, now the Chalk escarp- 

 ments, and the two outer ones formed of Lower Greensand. In 

 course of time the sea breached the Chalk ridges in a number of 

 places (the majority of which are now windgaps), and through the 

 gaps thus formed a great quantity of quartz-pebbles, etc., from 

 the Lower Greensand were carried." 



Those who are familiar — and what geologist is not ? — with the 

 old controversies of last century on the Denudation of the Weald, 

 will recognize here a return, in all essential points, to the " marine " 

 theory of the formation of escarpments and windgaps, which many 

 of us imagined to have been completely and finally overthrown 

 by the arguments of Greenwood, Eamsay, and other founders of 

 the " subaerial " school. The hypotheses advanced by these writers 

 were afterwards enlarged as our knowledge of river development 

 grew, and in their improved form have been applied by Professor 

 Gregory ' and Professor W. M. Davis ■ to the very region with which 

 Mr. Barrow deals, and it is therefore singular that he should discard 

 these views without giving his. reasons, and return to the old marine 

 hypothesis without any comment on the difficulties thereby incurred. 



It would take up too much space and serve no useful purpose 

 to recapitulate all the arguments by which that hypothesis was 

 opposed, but the two principal are — (1) that the postulated series of 

 narrow, parallel arms of the sea following the strike of the strata 

 has no counterpart in modern geography ; (2) that it affords no 

 satisfactory explanation of the origin of transverse rivers and their 

 constant relation to the dip. Bearing these objections in mirid, 

 let us see how far the subaerial theory is invalidated, and a return to 

 marine action compelled, by the new facts contained in Mr. Barrow's 

 paper. After tracing the small quartz joebbles of the various drifts 

 back to beds, now largely destroyed, of early (? Pliocene) age — the 

 " Pebble Gravel " of Mr. Whitaker — he shows that this material 

 must have come from the Lower Greensand, entering the Eocene 

 region through at least one of the windgaps of the Chiltern Hills ; 

 and he also expresses the opinion that the Thames Valley was invaded 

 by the sea at the same period. All this is quite compatible with the 

 subaerial hypothesis, but that does not satisfy Mr. Barrow, who 

 says,^ " Without going into minute details, it will be sufficient for 

 the present to say that this type of transit across the Gault could 

 hardly have taken place unless that had an almost even surface 



• Nat. Sci., vol. V, 1894, pp. 97-108 ; Geol. Mag., 1914, pp. 145-8. 

 ^ Gfiogr. Journ., vol. v, 1895, p. 145. 

 ^ Op. cit., p. 40. 



