
GEOLOGICAL SURVEYING 283 
posure—say, in sea-cliff, river-course, or railway-cutting. 
Although a soil be charged with abundant angular fragments 
of one and the same kind of rock, it does not necessarily 
follow that the parent rock from which these fragments have 
been derived, will be encountered immediately underneath 
the surface. Much will depend upon the configuration or 
shape of the ground. All soils and disintegrated rock- 
materials tend to travel downwards from higher to lower 
levels, and, in this way, soil derived from one kind of rock 
comes to overlap and to be commingled with soil derived, 
it may be, from quite a different class of rock-material. The 
annexed diagram (see Fig. 108) will serve to illustrate this 
point. Here there are three separate beds represented—a 

FIG. 108.—TRAVELLING OF SOIL AND SUBSOIL. 
being a dark red marl; 4,a grey sandstone; and c¢, a coarse 
conglomerate. The soil overlying a, which occupies the top 
of the hill or bank, is red and argillaceous, and as this soil 
tends to travel down the slope, it invades the outcrop of the 
stratum 04, where it becomes commingled with grey sand, 
derived from the disintegration of the sandstone. There is 
thus a gradual passage from a pronounced dark red clay soil 
into a more or less arenaceous soil of a lighter tint—the red 
colour gradually becoming less and less conspicuous as the 
base of the slope is approached. It is obvious that angular 
fragments of grey sandstone may be met with in the soil, at 
all levels from the outcrop of 4 downwards, while stones from 
the conglomerate will be confined to the soil that overlies that 
stratum. This commingling of soiland disintegrated rock-débris 
along the boundaries of formations is everywhere observable, 


