1882.] 83 [Davis. 



Lands of our Western Plains. The ice-sheet advancing over 

 these sometimes shoved off layers of very considerable size, but 

 the action seems to have been rather pushing than excavating. 

 Boulder clay was often forced into crevices thus formed. The 

 slabs do not seem to have been carried any great distance. 



This piece of evidence taken alone would imply great erosive 

 ability on the part of the ice, but its examples are excejotional 

 and when found occur close by others where no important 

 erosion has taken place, as is shown in the next argument. 



The Island of Moen in the Baltic, east of Denmark, offers the earliest 

 recognized examples of the kind ; it has often been described; Forchham- 

 mer considered the accompanying boulder clay eruptive, so closely does 

 it fill all the crevices in the broken chalk strata (Fogg. Ann., lviii, 1843, 

 614, 625) ; Puggaard attributed the contortions of the chalk to postgla- 

 cial plutonic forces (Geologie der Insel Moen, Leipzig, 1852, 61) ; this view 

 is accepted by Lyell (Antiquity of Man, 387), who copies some of Puggaard's 

 figures. Johnstrup first ascribed the dislocaticns to the pressure of an 

 advancing ice-sheet (Deutsch. Geol. Ges. Zft. xxvi, 1874, 533-585) : 

 this is adopted by Helland, and Penck (id., 1879, 71, 176.) The dislocated 

 strata are seen, greatly bent and arched, in the face of a cliff one to four 

 hundred feet high : although severely disturbed, it is not known that they 

 were carried forward. 



Remele describes a slab of chalk-strata in North Germany, one-quarter of 

 a (German) mile long (about 2000 metres), and 25 metres thick; breadth 

 not visible. (Deutsch. Geol. Ges. Zft. xx, 1868, 650-652.) 



Penck refers to several accounts of these slabs, and states that whole 

 quarries have been worked in them, and lime-kilns supplied from them 

 for many years, (loc. cit. 120.) 



J. Geikie refers to a boulder in Norfolk measuring 480 by 44 yards, and 

 to another in Lincolnshire, 430 by 30 feet. (Prehistoric Europe, 1881, 194.) 



B 4. Central and Marginal Areas of Glaciation. In examin- 

 ing next the arrangement and distribution of the drift, we must 

 divide the several glaciated regions into central and marginal 

 areas ; the first, mountainous or rugged ; the second, of much 

 smoother and lower surface. It will then be seen that in the 

 central areas the amount of drift is relatively small, as it is con- 

 centrated in the valleys, and does not cover the whole surface ; and 

 that it rests upon firm, striated rock from which all preglacial soil 

 has been swept away. On the other hand, in the marginal area, 

 the ground moraine forms an almost continuous sheet ; it rests 

 as a rule upon preglacial soil which was not rubbed away by the 



PROCEEDINGS B. S. N. H. VOL. XXII. 8 DECEMBER, 1882. 



