264 



EARTH FEATURES AND THEIR MEANING 



glaciation, but after the culminating point has been reached, the 

 period of amelioration of climate is the receding hemicycle. 



The advancing hemicycle. There is little reason to doubt 

 that whatever be the cause of the climatic changes which bring 

 on glacial conditions, these changes come on by insensible grada- 

 tions. The first visible evidence of the increased severity of 

 the climate is the longer persistence of the winter snows, at first 



FIG. 292. An Alaskan glacier spreading out at the foot of the range which 



nourishes it. 



within the more elevated districts. In such positions drifts must 

 eventually continue throughout the warm season and so con- 

 tribute to the snow accumulations of the succeeding winter. This 

 point once reached, small glaciers are inevitable, even should the 

 average temperature fall no further, for the snow left over in 

 each season must steadily increase the depth of the deposits until 

 the weight brings about an internal motion of the mass from higher 

 to lower levels. 



The inherited depressions of the upland the gentle hollows 

 at the heads of rivers will first be filled, and so the valleys 



