218 



KNOWLEDGE 



[October 1. 1898. 



other deposits of the Ice Age. Nothing like a true esker 

 could be quoted, moreover, from the sea-banks now forming 

 off our shores. The North Sea, at any rate, should have 

 given us some clue to their formation ; in the absence of 

 such evidence, the marine theory was adopted with con- 

 siderable reserve. 



Prof. SoUas,* in his valuable review of the history of 

 the subject, points out that Mr. N. 11. Winchell and Mr. 

 Warren Upham in America were among the first to show 

 how mounds and ridges of gravel could accumulate at the 

 base of crevasses in a glacier, and how they would come 

 to light on the final melting of the ice. But Mr. J. G. 

 Goodchild, on our side of the Atlantic, was at the same 

 time elaborating his views as to the distribution of 

 materials carried in the body of the ice — such materials as 

 we now call "englacial" or " intraglacial." In a paper 

 on the Eden Valley,! read in 1874, Mr. Goodchild gives 



Fig. 1. — View from the road along the Esker at Balrothery, 

 showing the low GraTel Eidge, and the Dublin Mountains in the 

 distance. 



the gist of the matter in this sentence : — " The angular 

 moraine-hke drift occasionally found in parts of the dales, 

 the upper and lower tills and the intercalated beds, the 

 deposits of sand and gravel that form the eskers, and, 

 finally, the numerous boulders that are left at nearly all 

 elevations, are each and all the results of the melting 

 of a great sheet of land ice that was charged throughout 

 with rock-fragments of all sizes and of all kinds occurring 

 within the area wherein the ice originated." Mr. Good- 

 child held that eskers were formed where materials in the 

 lower part of a melting ice-sheet were arrested by some 

 underlying ridge of rock. The water would run on either 

 side, and would leave a long bank of pebbles to mark the 

 line where scouring action was least, i.e., the line between 

 two adjacent subglacial streams. Simultaneously, Dr. 

 Hummel in Sweden was putting forward his view that 

 the eskers accumulated in the cliannels of such streams, 

 and that they are casts, in fact, of the grooves worn in the 

 bottom of a glacier by the streams that issue from it. 



* " A Map to show the Distribution of Eskers in Ireland," Sci. 

 Trans. R. Duhlin Soc, Tol. V. (1896), pp. 788 and 794. 



t Quart. Journ. Geol. fioc. Vol. XXXI. (1875), p. 99. See also 

 " On Drift," Oeo!. Uag., 1874, pp. 509 and 510. 



Good accounts of Hummel's paper are given by Prof. Jas, 

 Geikie^' and Prof. Sollas, and the former practically intro- 

 duced it to English readers. Dr. Hoist, in IHTO, urged 

 that pebble-accumulations in the beds of rivers on the 

 surface of melting ice may in time be lowered, by melting 

 and excavation, until they are left as ridges on the glacier- 

 floor, when this' finally comes to be exposed. Like (iood- 

 child. Hoist lays stress on the amount of intraglacial 

 material, which would fall out into the stream-channels as 

 they cut more deeply into the ice. 



In the last twenty years, as may be seen from Prof. 

 SoUas's review, opinion has favoured the explanation 

 given by Hummel rather than that of Hoist. Prof. 

 I. C. Kussell has, moreover, seen eskers in course of 

 formation in Alaska, t and to his account of the Malaspina 

 glacier we shall have occasion to return. His remark that 

 such deposits are typically associated with stagnant ice- 

 sheets which are wasting away, may account for the 

 disappointment felt by those observers who have failed to 

 find modern eskers under more normal types of glacier. 

 One might surely, however, have expected to meet with 

 sub-glacial eskers in Spitzbergen; yet Messrs. Garwood and 

 Gregory ; are obliged to report that evidence regarding 

 them is absent. 



These two authors, in their crisp, condensed, and 

 unspeculative record, give strong support to Mr. Goodchild's 

 theory of the importance of intraglacial drift. The waste 

 material of the highlands round about a great glacier- 

 basin falls upon the ice, and is gradually incorporated in 

 the mass. It is transported laterally as well as vertically 

 in the body of the ice, and becomes spread out into sheets, 

 forming intraglacial strata. Here and there, by internal 

 surging movements, it may become mingled with detritus 

 that has already been ground against the floor. Ultimately 

 it is extruded to form part of the copious clays, or sands, 

 or gravels of the terminal moraine. The stones have 

 generally been rounded, and are now attacked by the out- 

 flowing waters, and are re-arranged by their action at the 

 glacier-foot. In periods of shrinkage of the ice, when 

 melting has thoroughly set in, the intraglacial drift comes 

 rapidly into prominence. " Stratified sands and gravels " 

 are left behind in all the hollows ; vaUeys are choked, and 

 the striated floor and the roches moutotine'es are concealed as 

 quickly as they are deserted by the ice. In steeply falling 

 valleys, it is unlikely that an esker-ridge would escape 

 destruction during this final period of flood and flow. In 

 open plains, however, the case is very different. 



Let us picture Ireland in the lee Age — a time of 

 moderate coldness and abundant precipitation. The cold 

 was sufficient to cause all the moisture to be deposited as 

 snow ; the precipitation was aided, moreover, by the 

 greater height of the mountain-rim of the country, par- 

 ticularly on the western side. Above the town of Sligo 

 at the present day, the Carboniferous Limestone rises in 

 bold cliffs and plateaus ; and the enormous quantity of 

 limestone pebbles in the gravels of the plain shows that 

 such high masses must have been common at the opening 

 of the Glacial epoch. The plain itself was, however, 

 determined by the syncUnals of the Hercynian folding j ; 

 it had already assumed the character of a lowland, and 

 was no doubt covered in part by swamps and pools, on 



* " The Great Ice Age," 3rd edition (1894), p. 170. See also 2nd 

 edition (1877). 



+ Thirteenth Ann. Report. U.S. Geol. Surrey (1892), pp. 05 

 and 81. 



J "Glacial Geology of Spitzbergen," Quart. Jour. Geol. Soc. 

 Vol. LIT. (1898), pp." 211 and 222. 



§ Knowiedge, Vol. XXI., p. 78. 



