48 INITIAL CAUSES. 



The essential effects of glacial deposition are produced by the size and uni- 

 formity of the particles and by the place of deposit, i. e., on land or in water. 

 While fluvio-glacial deposits often show more sorting, glacial soils proper show 

 all possible variations. A till sheet may consist of gravel, sand, or clay, but 

 frequently of all three. It may contain pebbles, or boulders, or the deposit 

 may be largely made up of enormous blocks. The latter present the extreme 

 conditions for rock succession, while the till sheet proper offers an area pre- 

 pared for a higher type of colonists. The ratio of sand or gravel to clay deter- 

 mines the holard and echard of the till and the invasions upon it. This is 

 relatively immaterial when deposit occurs in water, but is significant in the 

 ordinary case of deposit on land, particularly where there is considerable 

 initial relief. Here the influence of slope upon water from the melting ice is 

 the same as upon ordinary precipitation. The ridges are drier, valleys wetter, 

 and the slopes intermediate, and the course of succession varies accordingly. 



Bare areas due to deposit by ice and snow. — ^The action of shore-ice is a 

 combination of erosion and deposit, though when a shore-wall is thus formed 

 it is a true deposit. Its structure, depth, and extent usually distinguish it but 

 slightly from the ordinary shore. In consequence, the development of vege- 

 tation upon it rarely produces any distinctive featiu-es. 



Deposit in consequence of snow action is confined to snow-slide masses and 

 to flat areas or hollows in which snow melts. In such snow-hollows the 

 deposit is usually insignificant, but the accumulation of the dust and sand 

 brought to the snow-field by wind often becomes appreciable in a few years. 

 In practically aU cases the real effect is produced by the partial destruction 

 of vegetation by the snow and the ponding of the snow-water. 



Snow-slides may be assigned either to snow or gravity, since they are due 

 to the combined action of both. They are more frequent than land-slides 

 of like extent, but they differ from them in few respects. A snow-slide sweeps 

 away the vegetation more or less completely, but may disturb the soil to a 

 sHght depth only. A heavy fall of snow may initiate a land-slide, however. 

 The mass of detritus at the bottom of a snow-sUde is much more homogeneous 

 and contains more plant material than do most land-slides. It differs also in the 

 fact that it may require one or more summers for the snow to melt. During 

 this time the mass remains cold and wet and invasion is correspondingly slow. 



Bare areas due to deposit by gravity. — ^Talus masses and slopes are universal 

 deposits at the base of cliffs, shores, banks, etc. From the nature of their 

 formation they differ chiefly in composition, apart from differences due to 

 location. The initial conditions presented are often very like those of the 

 cliff or bank above. The chief change is one of density or coherence. This 

 is well shown by the fact that the Uchens and cleft plants of a granite cliff or 

 wall are usually foimd in the talus as well, even when this shows the degree 

 of disintegration foimd in a gravel-sUde. Rock talus, in consequence, really 

 continues the pioneer stage begun on the fragmenting area. The develop- 

 ment is hastened by the more rapid weathering and the greater irregularity 

 of surface, which permit corresponding variations of holard. Talus derived from 

 soils such as sand or clay, or from rocks which decompose readily, presents 

 typically more extreme conditions as to water-content and stability than the 

 fragmenting area. The initial stages upon talus are hence new stages, and show 

 much less relationship to the population of the top of cliff or bank (plate 7 b). 



