Davis.] 38 [May 3, 



but not destroying the sands and gravels below. He describes the lower 

 till as containing the largest boulders ; the later ice-advances passed over a 

 smoother, drift-covered surface and found less opportunity to break off 

 opposing rock-ledges. (Geschiebeformation Norddeutschlands, Deutsch. 

 Geol. Ges. Zft., xxxi, 1879, 195, 198.) 



Berendt explains the lower till as the ground moraine of a sheet of land 

 ice; the interglacial gravels formed during a depression of the land when 

 the ice floated across from Scandinavia; the second till made as a second 

 ground-moraine when the ice settled down as the land rose again. (Gletscher- 

 theorie oder Drifttheorie in Norddeutschland. Deutsch. Geol. Gesell. Zft. 

 xxxi, 1879, 1-20.) We need not accept in full either of these theories as to 

 the motions of the ice- sheet, except so far as they show that it frequently 

 moved over loose sands and gravels without destroying them. 



A. Helland gives good evidence to show that Scandinavian ice actually 

 crossed over to North Germany, and that Avhile it had eroded a great 

 amount of rock near its source, its action had generally changed after pass- 

 ing the Baltic. Admitting that the so-called interglacial deposits may be 

 sometimes local or subglacial, he thinks it probable that there were two gla- 

 cial invasions, and explains that the second failed to destroy the interglacial 

 deposits because, while the Scandinavian Peninsula was an area of glacial 

 erosion, the North German Plain Avas an area of deposition, thus adding a 

 new point of similarity between glacial and river action. (Deutsch. Geol. 

 Ges. Zft. xxxi, 1879, 63—.) 



H. Credner calls attention to the preservation of preglacial river gravels 

 under the glacial drift near Leipzig, and to the general occurrence of strat- 

 ified drift below the till in Saxony. (Ueber die altdiluvialen Flussschotter 

 und die Diluvialhugel der Gegend von Leipzig, Deutsch. Geol. Ges. Zft. 

 xxxn, 1880, 588.) 



It would seem therefore that the first advance of Scandinavian 

 ice across the eroded land-surface of Tertiary strata in North 

 Germany in some places deposited a ground-moraine upon a bed 

 of gravel washed from the ice itself or of preglacial date ; and 

 in other places, where the form of the surface offered' more 

 resistance to the passage of the ice, the rock was broken off from 

 the projecting ledges, and shoved along a certain distance under 

 the ice. By this rubbing down and filling up of inequalities, 

 the old land surface was left smoother than before ; it was fur- 

 ther levelled by deposition of gravels from glacial streams 

 during any local retreat of the ice ; then the later advances of 

 the oscillating front found a more even surface for their motion, 

 and hence the later boulder-clays were laid down with less dis- 

 turbance on the loose detritus beneath. 



