Horace B. Woodward — The Chalky Boulder-clay. 489 



its materials to the Boulder-clay, as elsewhere such weathered rock 

 has been removed, and incorporated into the Drift. 



In parts of Norfolk where the bare Chalk comes to the surface, it 

 now weathers into a kind of rubble, as may be seen near Wells ; 

 and, land-ice overspreading such a surface, the superficial layers 

 would be frozen to its base, and, with the aid of a little Kimeridge 

 Clay, the Chalky Boulder-clay would be readily formed. During 

 the movement of the ice-sheet the lumps of chalk would become 

 scored by pressure against tiny fragments of shattered flint and 

 other hard rocks, along the shearing planes of the ice. As remarked 

 by Crosby, the detritus reaching the highest levels in the ice is 

 cari'ied farthest, and we may infer that such was the case with so 

 much debris of Chalk, the harder fragments of which have mostly 

 suffered faceting and striation. This would occur along the many 

 minor planes of movement in the ice which Chamberlin has so 

 clearly pictured.^ 



Thus, on the melting back of the ice, the further travelled Chalk 

 and flints became commingled with the more local debris ; and when 

 a subsequent movement of the ice took place, the ground-moraine 

 was pressed into the tough material of which the Chalky Boulder- 

 clay so generally consists, and this subjacent drift was itself disturbed 

 in many places, and dislocated as if by faults. 



It should be borne in mind that in certain areas, as in East Norfolk, 

 an earlier Boulder-clay, the Cromer Till, no doubt furnished some 

 erratics to the Chalky Boulder-clay which overspread it ; while other 

 travelled stones may have come from ice proceeding from western 

 and north-western areas overriding the ice of the Eastern-Midland 

 plain. Pebbles of quartz and quartzite may in part have been 

 derived from Tertiary gravels, as remarked by Sir Henry Howorth.^ 



In his account of Ice-work in Edeuside Mr. J. G. Goodchild long 

 ago maintained that Boulder-clay was not dragged along between 

 the ice-sheet and the rocks beneath, but that it was formed of the 

 weathered detritus of the rocks and other loose material, and de- 

 posited on the melting of the ice at or not far from the areas where 

 the Boulder-clay is now found. He also refers to the different 

 currents of ice that may have been present over an area under- 

 going land-glaciation, and to which causes many admixtures of 

 bouldei'S from diverse localities may be due.^ Other explanations 

 of the " intercrossing of erratics " have been given by Mr. Clement 

 Eeid and Mr. Lamplugh, who point out that stones from various 

 localities may have been scattered over the bed of the North Sea 

 by floating-ice prior to the invasion of the area by land-ice, and 

 many of these boulders would then become incorporated in the 

 subsequent ground-moraine.^ 



' See " Eecent Glacial Studies in Greenland " : Bull. Geol. Soc. America, vol. vi, 

 p. 199. 



- Geol. Mag. 1896, pp. 455, 463. 



^ Quart. Journ. Geol. Soc, vol. xxsi, p. 55; and Trans. Cumberland Assoc, 

 No. 11, lb87, p. 111. 



^ Eeid, "Geology of Cromer," p. 90 ; Lamplugh, Geol. Mag. 1890, p. 67. 



