470 THE FRESH- WATER LOCHS OF SCOTLAND 
the southern half of the North Sea forming a plain watered by the 
Rhine. The Yold'ia {Leda) niyalis bed, resting transgressively some- 
times on the Forest Bed and sometimes on the Wey bourn Crag, 
indicates a slight depression of the estuary, and the prevalence of 
boreal and arctic conditions. At the top of this sequence we find 
the arctic fresh-water bed with plant-remains, proving a great 
lowering of temperature, which, in the opinion of Mr Clement Reid, 
may have allowed the seas to be blocked with ice during the winter, 
and glaciers to form in the hilly districts. From these data it would 
appear that at this stage in Norfolk the relative position of land and 
sea must have been much the same as at the present time. Evidence 
tending to support this conclusion has been recently obtained in the 
south of L'eland. \\\ view of these data, there can be little doubt 
that the refrigeration of climate which culminated in the glacial 
period was a slow and gradual process. 
Before proceeding to describe the glacial phenomena of Scotland, 
we ought to call special attention to the fact that the main valley 
systems of the country and the dominant features of the High 
Plateau had been determined in pre-glacial time. The prolonged 
glaciation of the mainland and the outer islands produced important 
modifications of these physical features, which have survived to the 
present day. 
Throughout Scotland there is overwhelming evidence of the 
intense glaciation of the northern part of Britain. The phenomena 
point to (1) a period of maximum glaciation, when the Scottish and 
Scandinavian ice-sheets coalesced on the floor of the North Sea ; (2) 
a period of valley glaciers which became confluent in certain areas. 
Each epoch is characterised by difl'erent centres of ice dispersal, by 
diflerent methods of ice erosion, and by distinctive glacial accumulations. 
MAXIMUM GLACIATION 
During the period of maximum extension the ice must have 
enveloped the whole country and radiated from three great centres ; 
the first being situated in the Northern Block, to the north-west of 
the Great Glen ; the second, in the Central Block, between the Great 
Glen and the eastern border of the Highlands ; and the third, in 
the Southern Uplands. The ice-shed of the Northern Block ran 
approximately north and south, and over a large part of the area 
lay to the east of the present watershed ; that of the Central Block 
appears to have had a short axis trending generally east and west, 
and situated in the region of the Moor of Rannoch ; while that of 
the Southern Block ran in a north-east and south-west direction 
along the crest of the Southern Uplands, from Broadlaw in Peebles- 
shire to the Merrick in Kirkcudbrightshire. Beyond these main 
