PHYSICAL CONDITIONS— PLEISTOCENE. 351 



Channel -area — the borders of Northern France and Southern 

 England — do not appear to have subsided more than a few 

 fathoms below their present level. But the depression increased 

 towards the north or north-west, and seems to have reached its 

 maximum in Wales, the north-west of England, and in Ireland 

 After that maximum was attained a movement in the opposite 

 direction followed — the land began to emerge and the sea to 

 retire, and still the cold continued to increase. About this time 

 we know that the low grounds of Prussia were submerged by a 

 sea in which Leda {Yoldia) arctica abounded — a fact which may 

 lead us to suspect that Holstein, Denmark, and Scania, were like- 

 wise under water. To what extent the re-elevation of the 

 British area was continued we shall most probably never be able 

 to ascertain. All Ave know is that before it had attained even 

 to its present level, snow-fields and glaciers had already made 

 their appearance, and an arctic fauna lived round the shores of 

 Scotland. Steadily encroaching upon the low grounds, those 

 glaciers at last coalesced, while nappes of snow gathering upon 

 all the hills of lesser elevation gave rise to little ice-caps, which, 

 flowing down the slopes, gradually dilated upon the lowlands. 

 Thus in time all Scotland became enveloped in ice that flowed 

 west to break off in deep water beyond the Hebrides, and east 

 to meet the Scandinavian mer de glace which had all the 

 while been creeping outwards into the basin of the North Sea. 

 The north of England was likewise shrouded in ice — part of its 

 . sheet coalescing with the Scottish mer de glace in the basin of 

 the Irish Sea, and part with the united Scottish and Scandi- 

 navian ice -sheet that filled up the German Ocean. How far 

 south in England that ice-sheet flowed still remains to be more 

 rigorously determined. A broad belt of ice overflowed from the 

 basin of the Irish Sea, and, uniting with the glaciers that de- 

 scended from Wales, spread in the direction of the Severn Valley. 

 In like manner the ice that flowed eastward from the Pennine 

 Chainj to coalesce with the mer de glace of the North Sea, appears 

 to have advanced into Lincolnshire. But in the high grounds 

 of Derby the ice-flow may have been more or less independent, 



