Fisher — Denudations of Norfolk, 553 



an extensive spread. Since the whole of the area of Norfolk was probably deep 

 beneath the ocean which deposited this clay, and denudation consequently almost 

 suspended, our great ignorance of the period in question does not greatly affect the 

 subject in hand. 



The researches of Messrs. Wood and Rome in Lincolnshire and Yorkshire^ 

 have, however, thrown some light upon the series of deposits subsequent to the 

 Upper l^oulder-clay of Norfolk. The transport of ice-borne materials appears by 

 no means to have ceased with the Upper Drift of Norfolk. That deposit becomes a 

 fresh point of departure, and is described by them as the basemait Boulder-clay of 

 the adjoining county. There it is on the eastern or seaward side of the Wolds 

 overlaid by another Boulder-clay containing Chalk pebbles in much diminished 

 abundance, and called by them the Purple-clay. Patches of the same clay on the 

 northern and western confines of Yorkshire prove that that clay, though now gene- 

 rally denuded, once extended considerably to the north-west of its present boundary. 

 This clay occurs on the summit of the Chalk escarpment at a height of 400 feet j^ 

 and the authors conclude that the entire absence of Chalk in its upper portion is 

 due to the submergence of the entire Chalk range under a very deep ocean. There 

 may, I think, be another explanation of this fact ; for it does not follow that 

 glacial conditions must prevail wherever glacial deposits are carried ; and yet 

 again, a glacial envelope of a land surface is not a necessary consequence of a very 

 cold climate, great precipitation bemg also a requisite concomitant condition.^ 

 Either of these suppositions might perhaps explain the absence of Chalk debris in 

 the Purple-clay without necessitating the immense submergence supposed. 



Having thus traced the changes, with their accompanying denudations, which 

 built up the series of deposits between the fundamental Chalk of Norfolk and its 

 Upper Boulder-drift, and having pointed out from a reference to the work of 

 Messrs. Wood and Rome that there was probably a still further accumulation of 

 material, of which no remnant now remains in this county, I am next called upon 

 to suggest the means by which an immense quantity of these accumulations has 

 been removed, and the surface reduced to the undulating form which now obtains. 



There are, then, two grand points for consideration in this problem. First, the 

 removal of material, and secondly the formation of contour. 



But the formation of contour is due to the removal of material ; and hence the 

 question arises whether the same agencies which have produced the present contour 

 have removed all the material. Have the Purple-clay — which probably covered the 

 Upper Drift of Norfolk— and the Upper Drift, and the Middle Drift, and the Lower 

 Drift, been all denuded from the Chalk hills of this county by the same agency 

 which gave them their form, and carved out the valley in which we are now 

 assembled. It is demonstrable that such was not the case. 



Almost all geologists of the present day agree with Hutton and Playfair that the 

 winding valleys, and gently swelling hills, were not shaped by ocean waves and 

 currents. They are the result of subaerial as distinguished from submarine denu- 

 dation. But the whole pile of rocks, out of which they have been formed, was once 

 beneath the sea. The land must have been raised, or the ocean lowered, before 

 subaerial denudation could commence. During that process, whether it were short 

 or long, marine denudation must have taken place. Hence the questions are how 

 much of it was due to the sea ; and how did the sea act. I believe it may be 

 accepted as an axiom that under any given circumstances, whether of suljmarine or 

 subaerial denudation — that is, in all cases where the movement of material is 

 effected by the combined action of the pressure of moving water and gravity— a 

 state of comparative, not absolute, equilibrium will be soon attained. But when 

 an alteration of the relative levels takes place, producing a shallowing of the sea, 

 or an increased inclination of river courses or glacier beds, the eroding agents begin 

 to act at greater advantage, the condition of approximate equilibrium is destroyed, 

 and denudation is accelerated. I need not enter into the mechanics of this. It will 

 occur to every one considering the subject, how the currents of the ocean move 

 more rapidly over the bottom when the depth is diminished up to a certain limit,* 



1 Journ. Geol. Soc. vol. xxiv. p. 146. 2 Qgol. Journ. vol. xxiv. p. 149. 



s Sutherland, Arctic Regions, Journ. Geol. Soc. vol. ix. p. 302. 



4 But there is a point at which the friction of the bottom so checks the motion of currents, 

 that denudation ceases, as may be seen on a low shore when the tide runs out. 



YOL. v.— NO. LIV. 36 



