130 A. J. JIJKES-BEOWNB ON THE 



clay and partly across the Chalk, and the broken line indicating the 

 original slope of the latter. 



There remains another question to be considered, and this is the 

 importance or magnitude of the break above indicated between the 

 Chalky and Purple Boulder- clays. This break would appear to be 

 greater in the valley of the Ancholme than in Holderness, and it is 

 quite possible that part of the Purple Clay may be absent from the 

 former district. It is at any rate very likely that there was a 

 more continuous formation of Boulder-clays on the eastern than on 

 the western side of the Wolds, since the ice which formed them 

 clearly came from the east and north-east. It is also not 

 improbable that current-erosion may have been going on in the one 

 locality while deposition was taking place in the other. 



The thickness of the Hessle and Purple Clays on the eastern side 

 of the Wolds is certainly greater than that of the similar clay in the 

 valley of the Ancholme, and this also is in favour of the supposition 

 that some of the Purple Clay is wanting in the latter district. On 

 the borders of the estuary of the Humber the Purple Clay certainly 

 thins out westward, and allows the Hessle beds to rest on the Chalk. 

 The clay about Winterton resembles the Purple Clay, but that to 

 the southward near Brigg is certainly more of the Hessle type. 



Without discussing the physical conditions of the Glacial period 

 or the exact manner in which Boulder-clays were formed, I will only 

 indicate the general sequence of events which a consideration of all 

 the facts known to me has suggested. 



To my mind the frequent occurrence of stratified beds, and the 

 actual existence of marine shells at more than one horizon in the 

 Brown-clay series, is sufficient evidence that the area of deposition 

 was essentially marine, and that the materials were borne by marine 

 ice. 



The basement beds of the Glacial series of !N"orfolk give indi- 

 cations of continued depression and a gradual deepening sea, till the 

 whole of eastern and midland England was submerged, and Chalky 

 Boulder-clay was spread like a sheet over the whole surface. 



This submergence, one of at least 600 feet, must have tended to 

 ameliorate the climate, and to cut off the supply of Boulder-clay. 

 Eventually if the direction of the earth-movement was reversed and 

 gradual elevation ensued, surfaces of the grey Chalky Boulder-clay 

 would be exposed, first to current-erosion, and then to subaerial 

 detrition. 



With the partial upheaval of the land, however, glacial conditions 

 would return; Boulder-clay would again be formed, and this for 

 some reason seems to have been generally of a purple-brown colour. 

 It was formed more continuously on the eastern side of the Wolds 

 than on the western ; but the Wold hills may only have been at this 

 epoch a submarine ridge, or a peninsula of very slight elevation. If 

 the Wolds were above water during the formation of the Purple 

 Clays, there must have been another movement of depression so as 

 to allow the Hessle Clay to be deposited on surfaces which are now 

 nearly 400 feet above the sea. The extension of shore-deposits with 



