288 PREHISTORIC EUROPE. 



6. Sand, gravel, erratics, etc. — The melting of the last ice- 

 sheet resulted in the wide distribution of erratic blocks, and 

 the heaping-up of sand and gravel hills, which cover enormous 

 areas in all the low grounds of Northern Germany, Poland, etc. 



It may have occurred to the reader who has followed me so 

 far that the phenomena which have been attributed to the 

 action of this great mer de glace appear to be contradictory. 

 How, some one may ask, can we believe that the ice which 

 tumbled up the enormous chalk-masses of Mben could at the 

 same time overflow more or less incoherent deposits of sand and 

 gravel like those at Eixdorf without sweeping them entirely 

 away ? To which it may be replied, For the same reason that 

 we believe flowing water does in some places erode and exca- 

 vate, while in other places it accumulates the detritus that 

 results from its own denuding action. One may surely hold that 

 a certain deep ravine or glen, in the upper part of a river-valley, 

 has been excavated in the course of ages by the stream one sees 

 at the bottom, and at the same time assert, without the fear of 

 being considered self-contradictory, that the broad alluvia (over- 

 lying, it may be, incoherent marine strata), in the lower and 

 more open reaches of the valley, have been deposited by the 

 very same river that dug out the deep ravine above. What we 

 maintain is simply this : first, that, in regions where the erosive 

 action of the ice-sheet was great, little or no boulder-clay was 

 allowed to gather, and hollows of smaller or larger dimensions 

 were scooped out, when the nature of the ground was favourable 

 to that end ; and secondly, that, in places where the grinding- 

 power exerted was less, thick boulder-clay frequently accumu- 

 lated, and sub-glacial and interglacial beds were often preserved. 



The ice-sheet flowed, we cannot doubt, with a differential 

 motion : it must have moved faster in some places than in 

 others. In steep valleys and over a hilly country its course 

 would often be comparatively rapid, but very irregular — lagging 

 here, flowing quickly there — while in wide, open valleys that 

 sloped gently to the sea, such for example as those of the Forth 

 and the Tweed in Scotland, the whole body of the ice would flow 



