1872.] dawkins classipicatio:!^ of pleistocene strata. 437 



14. The Physical Eyidence as to Climate. 



The actual existence of two out of the three climatal zones 

 during the Pleistocene age is amply proved by the physical evidence 

 of the Pleistocene strata, and altogether irrespectively of the mam- 

 malia. The researches of many eminent observers prove that after 

 the close of the Porest-bed era the temperature of Northern Europe 

 gradually became lowered, until it became arctic in character, and 

 the complex phenomena were manifested which we call glacial — 

 the deposit of clays with large angular and scratched blocks, the 

 grooving, polishing, and rounding of the tops of hUls and of the 

 bottoms of valleys, and the accumulation of marine gravels. 



At the commencement of the Glacial period in Britain an enormous 

 ice-sheet, similar to that under which Greenland now lies buried, 

 extended from the hills of Scandinavia over North Germany, the 

 North Sea, Scotland, Ireland, Cumbria, and the lower portion of 

 England, as far south as the line of the Thames valley, leaving the 

 grooved 'hill-tops and the Boulder-clay to show its extension *. The 

 land then most probably, as Mr. God win- Austen, Prof. Eamsay, and 

 Sir Charles Lyell believe, stood higher than it does now. To this 

 succeeded a period of depression, during which the mountains of 

 "Wales and the hills of Derbyshire and Yorkshire were submerged to 

 a depth at least 1300 feet below their present level ; and the waves 

 of the sea washed out of the Boulder- clay the shingle and sand which 

 constitute the "middle drift" of the north of England, Scotland, 

 and Ireland f. Then the region north of the Thames vaUey was 

 reelevated, and a period of glaciers set in, which, however, were 

 of far less extent than those which preceded them, occupying iso- 

 lated areas, and not uniting to form a continuous icy mantle to the 

 country. There may have been, and probably were, many more 

 changes in climate and geography than these three ; but these are 

 clearly and definitely marked in the whole of Britain north of the 

 valley of the lower Thames, while the subdivisions are not so clearly 

 traceable, and may be mere local phenomena $. 



Prom this it follows that there were at least three well-marked 

 changes in the Pleistocene climate of Great Britain, if not of Northern 

 France. The temperature of the Eorest-bed era was lowered until 

 it reached a minimum that found expression in the great continuous 

 ice-sheet moving resistlessly down to the sea, over the lower hills 

 and valleys, and traversing hills as high as 2500 feet §. 2ndly. Then, 

 coincident with the marine depression, the climate became warmer, 

 until certain southern moUusks, such as Cardium rusticum, found 



* Sir Charles Lyell has giren an admirable summary of the glacial phenomena 

 of Europe in ' The Antiquity of Man.' 



t For information regarding the Irish Drift I have to thank Mr. Kinahan, of 

 the GTeological Survey of Ireland. See also papers by Harkness and Hull, Geol. 

 Mag. vols. vi. & viii. 



I On this point see Mr. J. Geikie, Geological Magazine, No. 93, " On Changes 

 of Climate in the Glacial Epoch." 



§ According to Jamieson, the top of Sohehallion (Perthshire) is traversed by 

 strise which pass over from a northerly direction. Quart. Journ. Geol, Soc. 

 xsi. p. 162. 



