bound cavity, but in descending those portions of the 

 declivities B B and CO' which lie above O and O' the ice 

 gains considerably in strength, there is no support to the 

 rock structures owing to the parabolic tendency of the 

 glacial motion am I blocks are easily detached from the rock- 

 face. Therefore recession of the declivity at various points 

 ensues, and this recession takes place rapidly but with 

 varying speeds. 



'"Steps" will therefore be formed by the recession of the 

 profiles B B', O C and of B. "Treads, " are formed by the 

 recession of the stops and a ''thalweg" such as that illus- 

 trated in Pig. 14 (b) is formed at an early stage. An 

 important point, is here disclosed. It has been shown that 

 in a deep constricted valley a large glacier during its 

 youthful attack will form a deep and tlattish floored basin 



great depth and relative speed of the glacier [<, maintained, 

 and this implies a relatively large vertical measure of cor- 

 rasive strength. In the case of the recession of a " step" 

 on a steep channel slope however, the abrasive force of the 

 glacier at the foot of the "step" is very great, and there- 

 fore tends to the formation of basins at such points. But 

 on the other hand the "step" recession is relatively rapid, 

 and before the glacier has had time to excavate a deep 

 basin on the "tread" the "step" has receded, because of 

 the ease with which the structures forming the declivity 

 are broken up on the unsupported aspect of the "step." 

 Bj this rapid recession of the "step," the channel cross- 

 section as rapidly increases and this implies a corresponding 

 decrease of glacial velocity, with proportionate decrease 

 °f glacial strength. Therefore the "treads" of "steps" 



