THE DATE OF THE GLACIAL PERIOD. 357 



the Glacial period was, even in the north of France, a 

 time of great precipitation and of a considerable degree of 

 cold, when ice formed to a much greater extent than now 

 upon the surface of the Somme. The direct evidence of 

 this consists in the boulders mingled with the high-level 

 gravel which are of such size as to require floating ice for 

 their transportation. 



In addition to the natural increase in the eroding 

 power of the Somme brought about by the increase in its 

 volume, on account of the greater precipitation in the 

 Glacial age, there would also be, as Prestwich has well 

 shown, a great increase in rate through the action of 

 ground ice, which plays a very important part in the river 

 erosion of arctic countries, and in all probability did so 

 during the Glacial period in the valley of the Somme. 



" When the water is reduced to and below 32° Fahr., 

 although the rapid motion may prevent freezing on the 

 surface for a time, any pointed surfaces at the bottom of 

 the river, such as stones and boulders, will determine (as 

 is the case with a saturated saline solution) a sort of crys- 

 tallisation, needles of ice being formed, which gradually 

 extend from stone to stone and envelop the bodies with 

 which they are in contact. By this means the whole sur- 

 face of a gravelly river-bed may become coated with ice, 

 which, on a change of temperature, or of atmospheric 

 pressure, or on acquiring certain dimensions and buoyancy, 

 rises to the surface, bringing with it the loose materials to 

 which it adhered. Colonel Jackson remarks, in speaking 

 of this bottom-ice, that ' it frequently happens that these 

 pieces, in rising from the bottom, bring up with them 

 sand and stones, which are thus transported by the cur- 

 rent. . . . When the thaw sets in the ice, becoming rot- 

 ten, lets fall the gravel and stones in places far distant 

 from those whence they came.' 



" Again, Baron Wrangell remarks that, ' in all the more 

 rapid and rocky streams of this district [northern Siberia] 



