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PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Nov. 20, 



is more rolled, trie stratification more decided ; scratched fragments 

 more rare, if not entirely wanting ; while false bedding indicates the 

 pushing action of water, in seas of no great depth. The deposit, 

 however, still furnishes some erratic blocks and several beds of un- 

 mixed and unabraded chalk, indicating the action of floating ice. 

 This difference in the character of the upper and lower erratics may 

 arise, in part from the former having been deposited in a more open 

 sea, and in part from the mitigated rigour of the climate, particularly 

 during the deposition of the more recent portions of the upper 

 erratics. 



9. The contortions in the strata of Cromer Cliffs, which have 

 been referred to various causes, and among others to the ploughing 

 up of the bed of the sea by icebergs, appear susceptible of a better 

 explanation by supposing masses of ice fixed in the boulder clay, 

 which, as the coast subsided, became covered with laminated clay and 

 thin alternating beds of sand and gravel (see infra, p. 30, and Dia- 

 grams, figs. 3-5). As the ice melted, on the return of a milder climate, 

 these beds of clay, sand, and gravel would subside into the cavities 

 left in the till. When the area of the ice was extensive compared 

 with its depth, the result would be merely a slight curvature of the 

 strata above the till. When the thickness of the ice was considerable 

 compared with its area, the subsidence of the strata above it, com- 

 bined with the collapse of the walls of till bounding the cavity, would 

 produce every variety of contortion seen in the Cromer Cliffs. The 

 difficulty of fixing buoyant ice under water is no objection to the 

 hypothesis ; for the records of the Polar Voyages give to ice so fixed 

 the character of a vera causa. Sir Edward Parry found for miles 

 along the coast, near Melville Island, a dark blue stratum of solid ice 

 imbedded in the beach, at the depth of 10 feet under the surface of 

 the water. "The ice," he says, "had probably been the lower part 

 of heavy masses forced aground by the pressure of the floes from 

 without, and still adhering to the viscous mud of which the beach is 

 composed after the upper surface has in the course of time dissolved." 



10. The different elevations at which the boulder clay occurs on 

 the coast and in the interior are in accordance with the theory of the 

 gradual advance of an arctic littoral deposit over subsiding land, now 

 restored by re-elevation to about its former level ; and the theory will 

 explain the transport of large blocks from lower to higher levels. On 

 the eastern coast, where the united thickness of the upper and lower 

 erratics has been estimated at between 300 and 400 feet, that of the 

 boulder clay rarely exceeds 60 feet, varying not unfrequently to 

 1 feet ; while at Swaffham it has been ascertained that a little below 

 the summit of the watershed it fills a hollow in the chalk to the 

 depth of 90 feet ; and on the actual summit the upper erratics, which 

 are so thick on the eastern coast, rarely attain a thickness of 30 feet, 

 and are generally much thinner, usually resting on the chalk, but oc- 

 casionally on patches of till in its hollows. 



1 1 . While the eastern side of Norfolk best exhibits the history of 

 the erratic tertiaries during the period of depression, their history 

 during the period of elevation is best traced on the western side. On 



