484 PLEISTOCENE. 



form of Glaciers, Ice-sheets, or Coast-ice. It often exhibits 

 obscure horizontal streaks of different materials, which at first 

 sight might be taken for lines of bedding, but this " fluxion- 

 structure," ' as it has been termed, is usually formed of crushed 

 material, and is probably due to the pressure of moving ice.* Here 

 and there, as on the Norfolk coast, beds of laminated and even 

 ripple-marked clay and marl occur between beds of Boulder Clay ; 

 and deposits of this nature, as remarked by Mr. Reid, may be in 

 reality glacier-mud, such as would flow from beneath the ice, and 

 be deposited during its temporary retreat. 



The Glaciers which accumulated on the higher grounds of Britain merged in 

 some instances to form Ice-slieets {mer dc glace) ; and the thickness of these has 

 been reckoned at hundreds and even several thousands of feet : but probably an 

 estimate of from 400 to 2000 feet would be sufficient.^ Be this as it may, there 

 can be no doubt that considerable areas of ice were formed by the amalgamation 

 of Glaciers, and these were forced over the land, and sometimes pushed out to sea. 

 In the latter case they would break off in the form of ice-bergs, and eventually melt 

 and deposit the mud and boulders stored up in the ice and in crevices of it. Much 

 material so laid down would be more or less sorted and arranged in layers, but in 

 some instances where the ice-bergs grounded, the deposits would be disturbed. 

 The direction of the ice-sheets which, as a rule, radiated from the dominant high 

 grounds, was often modified by local causes, and perhaps also by the incoming of 

 ice-flows from Scandinavia. While the detritus or stony material brought down 

 by glaciers and deposited by the melting ice is known as Moraines, that which was 

 formed under the land-ice is known as the Ground-moraine, Bottom-moraine or 

 Moraine projonde, and this varies to a considerable extent according to the nature 

 of the rocks over which the ice-sheets travelled. Over hard rocks the ice leaves 

 its marks in the form of grooves and strise ; over softer strata the beds are much 

 crumpled and disturbed. The stride and the transported boulders indicate the 

 directions from which the ice came. 



Unfortunately, we have very little information concerning what kinds of accumu- 

 lations are being formed by land-ice in the arctic regions. There the icy agents 

 can be studied ; but the effects are not, as a rule, so conspicuous. In this country 

 we see the results, and have to infer what were the agents. 



In the formation of our Chalky Boulder Clay, for instance, icebergs are 

 generally admitted to have had little or no influence, for the ice from which the 

 floating masses would be severed must have passed over considerable tracts of 

 Chalk, and the Chalk would have been submerged if the icebergs were to have fair 

 play. Mr. Wood advocated the view that the Chalky Boulder Clay was extruded 

 at the foot of an ice-sheet that enveloped the Yorkshire and Lincolnshire Wolds ; 

 but the general absence of stratification in it goes against the view of this clay having 

 been for the most part deposited under water. He remarked that if we consider 

 the soluble nature of Chalk, it must be evident that none of this material can have 

 been detached from the parent mass either by water-action, or by any other 

 atmospheric agency than moving ice.'* We require, too, an agent capable of 

 exerting a comparatively uniform influence over wide tracts, of abrading and 

 pushing on material in a manner more extensive than could be attributed to coast- 

 ice. Moreover, the Clay has been much compressed, and is so tough and hard in 



1 Hugh Miller, jun., R. Phys. Soc. Edin. viii. 157. 

 "^ C. Reid, Geol. Cromer, p. 90. 



* The thickness of the Arctic and Antarctic ice-sheets must in places be several 

 thousand feet, judging from the size of some of the ice-bergs, especially in the 

 latter region. The Alpine glacier-ice is from 100 to 600 feet thick. J. Geikie, 

 Outlines of Geology, 1886, pp. 53, etc ; C. Reid, G. Mag. 1881, p. 234 ; see also 

 the Duke of Argyll, Address to Geol. Soc. 1873 ; Sir J. W. Dawson, Address to 

 Brit. Assoc. 1886, p. 27 ; and H. Carvill Lewis, Nature, Nov. 25, 1SS6, p. 89. 



* Q. J. xxvi. 100 ; Wood and Rome, Q. J. xxiv. 167. 



