Fisher — Denudations of Norfolk. 549 



The lower Boulder-clay overlies the laminated beds. But during the interval 

 between them the sea must have become much deeper, and have been now in- 

 volved in a system of extensive tidal currents. 



Ice capable of transporting mineral matter and depositing it at the bottom of 

 the sea occurs under two modifications, — as icebergs and as coast ice. Icebergs 

 receive the principal part of their freight while in the parent glacier, partly from 

 rocks falling on them, and partly from debacles from lateral valleys,^ and occa- 

 sionally, after they become detached, by approaching close to cliffs and receiving 

 falls from them. 



Coast-ice is that which forms by the freezing of the surface of the sea along the 

 shore. It is attached more or less to the land, and in that respect differs from 

 pack-ice, which is formed on open water. By pressure taking place among coast- 

 ice, heaps of shingle, sand, and mud are formed, to which the ice becomes fixed, 

 and the whole floated away. But a most important mode by which mud is col- 

 lected upon shore-ice, is when the thawing snow of spring brings down quantities 

 of mud from the land- and spi'eads it out upon the coast-ice before it breaks away. 

 When this afterwards takes place it is carried into deeper water and there deposited. 



It is clear that the deposits of small material from all these causes will take a 

 roughly stratified arrangement, such as we see in the lower Boulder-clay ; and the 

 smallness of the boulders contained in it and its gradual deposition appear to 

 point to coast-ice as the instrument of its formation. By coast-ice, also, the few 

 shells which it contains were picked up from off the shore. Some portions of the 

 mass have doubtless come from far, but probably the greater portion of it from 

 near at hand. In a fragment of this stratum which remains in the upper part of 

 the brickpit near the Norwich ferry, the clay seems to be chiefly derived from the 

 London clay, and contains numerous Eocene pebbles. 



What appears to be this lower Boulder-clay may also be observed in the low 

 cliff immediately above the Railway Station at Hunstanton, while a section given 

 by Mr. Rose^ proves that it extends to Lynn, where it rests"on the Oxford clay. 



Hence it seems that the area of the Wash, and of the low lands of part of the 

 Bedford level, were occupied by the sea in the early part of the Boulder-clay 

 period. It is on account of the antiquity of these master valleys that the lower 

 Boulder-clay takes the form of a coast deposit, owing to its low position, and this 

 gives it the (as I believe) fallacious appearance of bearing to the Upper Drift the 

 same relation that the low level gravels are supposed to bear to the higher, the 

 lower being the newer of the two. 



I am not aware of there being any decided proof whether or not the Chalk land 

 was covered by the sea at this period ; for I do not know that we find the lower 

 clay reposing upon the Chalk in any of the higher parts of the county. 



The occupation of the sea by coast-ice, to the exclusion of large bergs, points to a 

 condition of shallowness, and to a freedom from the currents of an open sea. 

 When the sea became deeper bergs might be expected to be introduced, and 

 accordingly we find unmistakable evidence of their action tow ards the close of the 

 deposition of the Lower Drift. 



I believe that that deposit had been a gradual and a tranquil one, and from 

 some change in the mode of deposition, possibly from an amelioration of climate, 

 it had become more sandy and more regularly stratified in its upper part, and 

 that at that period the sea had attained a considerable depth. Then the 

 period of the Middle Drift set in. Larger masses of ice, loaded with heavier 

 freights of sand, and gravel, and chalk, were introduced, and by their falling 

 upon the previously horizontal strata, those contortions were formed which at 

 first sight appear so perplexing in the Cromer cliffs. 



Let me request your attention to the juodits operandi. If a mass of material is 

 spread out equably over another, both being in a plastic state, the surface of their 

 junction will be horizontal. But if a quantity of one such material be dropped 

 upon a limited space of the other, it will sink down into it, and the depth to which 

 it sinks will depend upon the quantity which falls. The material of the lower 

 stratum, which is squeezed aside by the intrusion, will be driven into folds all 



1 Sutherland's Journal, vol. i. p. 127, 



2 Ibid. vol. ii. pp. 182, 198, and Journ. Geol. Soc. vol. ix. p. 308. 

 * Geology of W. Norfolk, Phil. Mag. vol. viii. p. 35. 



