304 I'HE BEDSHIEL KAIMS 



temperature, under geographical conditions such as these, 

 sufficed to permit of the snows on the higher ground 

 becoming perennial. Hence the climatal conditions arising 

 from the elevation of the land gradually changed from bad 

 to worse. More snow fell on the uplands each winter than 

 the summer's heat sufficed to melt, and the surplus pre- 

 cipitation began to flow oif the surface as glacier-ice instead 

 of water. Once geographical conditions like these are set 

 up, they tend to go on, and to affect an increasingly large 

 area around. 



It has already been stated that the greater part of the 

 Sun's rays, whatever their intensity, are nearly incompetent 

 to melt j(;tire snow and ice, and, of course, are quite 

 incapable of raising the temperature of either of these above 

 the freezing point. The air is warmed, not by the direct 

 heat of the Sun, but entirely by heat radiated back from 

 the surface of the earth ; hence the atmospheric temperature 

 rises above the freezing point only where the Sun's rays 

 happen to fall upon rock surfaces which are not covered by 

 ice and snow. 



It followed as one of the consequences of these principles 

 that, during the Glacial Period, the aqueous vapour drifting 

 upon the land from the Gulf Stream was chilled into fog 

 where it first came into contact with the lowlands, and 

 congealed into snow as soon as it reached the mountains. 

 Thus conditions such as occur in Labrador, or in Greenland, 

 gradually set in. Glaciers, once they were started, increased 

 in size ; little by little they crept outward from the hearts 

 of each mountain area in the direction of the lowlands, 

 following the old lines of depression as they did so, and 

 transporting enormous quantities of the rock material, which - 

 had been loosened and disintegrated by pre-glacial weather- 

 ing, down the valleys and on to the lowlands beyond. 

 By slow degrees the smaller glaciers became confluent, and 

 coalesced into larger masses. These in their turn united 

 into icy masses of larger proportions still ; until, in the end, 

 each great mountain area, slowly sending forth its own 

 contributions, and adding them to those of the neighbouring 

 masses, helped to swathe the whole of Northern Britain in 

 one vast mantle of ice. 



