﻿1850.] TRIMMER ON THE ERRATICS OF NORFOLK. 21 



tensive than any other tertiary deposits of Britain, although in some 

 localities they have been much interrupted by denudation. 



4. They occur under the form of an upper and a lower deposit, 

 possessing certain common characters, and certain others that are 

 distinctive. Boulders transported from a distance are found in both ; 

 but the lower erratic tertiaries deviate more from the type of other 

 tertiary strata than the upper erratics. 



5. The lower deposit or boulder clay was a littoral deposit of an 

 arctic climate, which advanced southwards during the subsidence of 

 the land, and retreated northwards during its subsequent elevation. 

 An examination of the soundings recorded in the Polar Voyages, par- 

 ticularly those of Sir Edward Parry, proves that in frozen seas mud, 

 which, under ordinary conditions, is regarded as a deep-water deposit, 

 is characteristic of the vicinity of land, where sand and shingle would 

 prevail in other seas. 



6. The position of the lower erratic tertiaries in the valleys proves 

 that the latter were excavated previous to the subsidence of the 

 glacial period, and indeed before the epoch of the mammalian crag ; 

 so that the general configuration of the land was nearly the same, 

 during the ante-glacial subaerial period, as at present ; old excava- 

 tions having been filled during the process of subsidence, and re-ex- 

 cavated, more or less, during the period of re-elevation. 



7. The distribution of foreign matter in the erratic tertiaries of 

 Norfolk is such as would have resulted from the action of shore-ice 

 on sinking land ; the ice being sometimes fixed to the coast for 

 months, and even years together, and sometimes in daily rapid and 

 capricious motion, produced more by winds than by tides (see the 

 Polar Voyages) ; the local action being modified, and the local and 

 foreign detritus blended, by a constant general current from the 

 north. The prevalent lines of transport in Norfolk are from the 

 north-east and from the west. Scandinavian erratic blocks are more 

 abundant on the eastern side of the watershed and the borders of the 

 German Ocean, while blocks and small detritus of oolitic rocks, in- 

 creasing in quantity westward along certain lines, indicate that quarter 

 as their source. They appear to have travelled chiefly along the valleys 

 of the Waveney and Little Ouse, which flow eastward and westward 

 from sources within a few yards of each other. 



A very slight depression would convert these valleys into a strait, 

 communicating with the Wash and insulating the greater portion of 

 Norfolk. But, although the general lines of transport have been 

 from the north and west, there is an occasional intermixture of de- 

 tritus borne in opposite directions under the combined influence of 

 the shore-ice, acted on chiefly by winds, and the changes of the con- 

 figuration of the surface, — valleys, as the land subsided, having been 

 converted into straits and friths, and hills into islands and promon- 

 tories. Near the heads of valleys, the boulder clay consists almost 

 wholly of materials derived from the bounding rocks ; while nearer 

 their mouths, it is much mixed with detritus derived from a distance. 



8. In the upper erratic tertiaries the phsenomena of ordinary ma- 

 rine action are more prevalent than in the boulder clay ; the gravel 



