THE GLACIAL PERIOD. 201 



this confusion was due to movements of the earth's crust — to 

 convulsions and "faults" caused by the action of the subterranean 

 forces, and in this view he was followed by Lyell. But Johnstrup 

 has since reinvestigated the evidence and come to quite a different 

 conclusion. He shows, in his interesting papers already referred 

 to, that the disturbances can only be attributed to the enormous 

 pressure and disrupting force of the Scandinavian mer de glace, 

 which filled up the basin of the Baltic and overflowed Denmark. 

 Chalk is just one of those rocks which would be most readily 

 ruptured and displaced under the crushing weight of the advan- 

 cing ice-sheet, and many good examples of this striking pheno- 

 menon have been recorded. Chalk boulders of large size are met 

 with in many districts in Denmark, Holstein, and Germany. 

 Thus Bruhns describes a chalk erratic in the boulder-clay of 

 East Holstein (Pariner Berg), which measured 86 feet in length, 

 10 feet in breadth, and 12 \ feet in thickness. 1 But even harder 

 and less easily ruptured strata than chalk occasionally show a 

 highly-broken surface below till. Thus, the limestone (Muschel- 

 kalk) at Biidersdorf, near Berlin, is smoothed and striated in 

 some places, while in other places it is much broken up, and 

 the shattered debris and displaced blocks are incorporated in the 

 bottom-part of the boulder-clay. 2 Similar appearances are met 

 with in the till that overlies the hard Silurian greywacke* of 

 Saxony, as we shall see presently. But the phenomena cer- 

 tainly occur on the largest scale with such strata as chalk and 

 the various Tertiary formations, which yielded more readily to 

 the pushing and crushing of the ice-sheet. Thus, at Teutschen- 

 thal, near Halle, in Saxony, the boulder-clay is described by 

 Helland 3 as appearing often like veins and patches in the Brown 

 Coal formation, underneath the main mass of the boulder-clay — 

 the Tertiary strata are frequently bent and broken, the coal-beds 

 being sometimes caught up and included en masse in the till. 

 Here and there, also, large detached fragments of the Tertiary 

 beds appear scattered through the boulder-clay in the same 



1 Zeitschr. deutsch. geol. Ges., Bd. i. p. 111. 

 2 Penck. Op. cit. 3 Zeitschr. deutsch. geol. Ges., 1879, p. 72. 



