GLACIAL BEDS. 485 



places that navvies engaged in making railway-cuttings have remarked that they 

 never had a more troublesome material to deal with. 



The occurrence of the chalk lumps in the clay may perhaps be accounted for in 

 the following way. The surface of the Chalk in Norfolk, along the sea-margin, 

 and in places inland, is seen to have weathered in a very rubbly form, to a depth 

 sometimes of six or eight feet. Frost and rains have no doubt been the chief 

 influences in producing this; and this weathered Chalk would furnish "ready 

 made " the very material for the Chalky Boulder Clay, which consists so largely of 

 lumps of Chalk which have not necessarily been rounded by rolling. 



It has indeed been considered that most of the material forming Boulder Clay 

 was obtained ready made, from the disintegrated surfaces of the strata. During 

 later Tertiary times, a great part of the country was dry land, and then no doubt 

 much "head" or subaerial detritus was formed; for, as observed by Prof. J. 

 Geikie, in all the glaciated countries of Europe there is a general absence of those 

 thick sheets of " weathered rock " which are so conspicuous a feature in regions 

 which have not been suljjected to glacial action.^ 



The term Esker is used in Ireland, and Kame in Scotland, to denote certain 

 peculiar ridges and mounds of drift which rest sometimes on the Boulder Clay and 

 other Glacial Beds, and regarding whose origin there has been much diversity of 

 opinion. Some geologists maintain that they have been formed by marine agency ; 

 some again consider that they are due to the action of sub-glacial waters (waters 

 flowing underneath land-ice), or that they are morainic accumulations deposited 

 either on land, or beneath the sea ; while yet others have attributed them to 

 torrential or fluviatile action. They occur often in long winding ridges, and rise 

 boldly and sharply with steep slopes, to heights of 100 feet or even more. As 

 remarked by Prof. A. H. Green, the gravel is very distinctly though irregularly 

 bedded, and the beds arch over, so that in a general way the direction and amount 

 of the dip is about the same as the slopes of the surface of the ridge. No one 

 explanation will account for all Eskers, but the majority were probably formed 

 where a river with fall enough to enable it to carry down gravel entered a tidal sea.'- 



During the Glacial period many of the old river-courses were completely choked 

 up with clay, stones, and gravel, so that when the ice melted away, the rivers did 

 not always, and in the north of England not often, regain their old channels. 

 Much Boulder Clay thus is found overlying old river-gravels, and it is sometimes 

 arranged in ridges, "drums" or " drumlins " (dnun lines). These clayey 

 mounds are usually formed of boulder clay interstratified with undisturbed beds 

 of sand and finely laminated clay, and occasionally they pass into Eskers in the 

 lower parts of valleys. Some river-gravels, when traced to their sources in the 

 mountains, merge into moraine-debris. 



There are certain phenomena known as 'Crag and Tail'; these terms are 

 applied to a bare scarp or boss of rock (Crag) which has superficial deposits 

 banked up against it on the other side, forming a long sloping "tail." These 

 appearances are due to the ice being obstructed in its passage by the abrupt hill of 

 hard rock, which permitted the accumulation of rock-fragments on the unexposed 

 side. 



The effects of Coast-ice must not be neglected in any attempt to elucidate 

 Glacial phenomena. In high latitudes an " ice-foot " or belt of ice is formed along 

 the shores, and sometimes reaches a height of 30 or 40 feet. Many fragments of 

 rock are frozen into the bottom of the ice, and it is also liable to receive accumu- 

 lations of detritus due to landslips and the weathering of cliffs. Eventually this 

 coast-ice is broken up, and large masses of it may be driven on shore, crushing and 

 grinding the rocks over which they are pushed, and striating them as well as many 

 of the loose stones.^ The Hessle Clay of Yorkshire and Lincolnshire may have 



' Outlines of Geology, 1886, p. 373; A. Geikie, Text-Book of Geology, pp. 

 325, 894 ; see also P. Geol. Assoc, i.x. 123 ; F. J. Bennett, Proc. Norwich Geol. 

 Soc. i. 256. 



* Physical Geology, 1882, p. 632. 



^ See Prof. John Milne, G. Mag. 1876, pp. 304, 403 ; Q. J. xxxiii. 929 ; and 

 A. J. Jukes-Browne, Physical Geology, 1884, p. 131 ; Geol. East Lincolnshire 

 (Geol. Surv.) ; Penning and Jukes-Browne, Geol. Cambridge, pp. 115, 117. 



