
GEOLOGICAL SURVEYING 287 
by the form of the ground. It is obvious, indeed, that the 
occurrence of a thick bed of relatively hard rock intercalated 
in a series of softer or more yielding strata, inclined in one 
and the same direction, must, under the influence of denuda- 
tion, give rise to the formation of escarpments or ridges— 
which, whether the naked rock be actually exposed or not, 
will form prominent features in a landscape. In the case of 
countries which are built up of horizontal strata, the varying 
hardness of the rocks will similarly affect the form of the 
ground, and cause it to assume a terraced aspect—a structure 
illustrated in the same diagram (Fig. 110), where the gentler 
slopes correspond with the outcrops of “soft” rocks, and the 
more abrupt gradients with the outcrops of “hard” rocks. 
It must be borne in mind, however, that in countries 
heavily covered with glacial and other superficial accumula- 
tions, the surface configuration of the underlying solid rocks 
is often obscured or even entirely concealed. But when such 
deposits are either absent or attain no great thickness, 
the form of the ground is always of the greatest assistance to 
the geologist who is trying to carry a line of outcrops across a 
country. 
(d) Sprvzngs.—Considerable aid in tracing boundary-lines 
is sometimes afforded by springs. When layers of relatively 
impervious materials, such as shales, clay, etc., are intercalated 
among a series of porous strata, underground water tends to 
come to the surface along the line of junction between the 
porous and the non-porous strata. This will often happen 
when bedded rocks are truncated by the slope of the ground, 
the water appearing in the form of springs or oozing out 
slowly and giving rise to marshy and damp spots. Should a 
number of such springs or seepage-places occur in succession 
in some given direction, they will necessarily indicate the 
presence of a geological boundary-line. Occasionally, spring- 
water is highly charged with mineral matter, such as 
carbonate of lime, iron-oxide, etc, and hence deposits of 
ealerssinter, bog-iron ore, ete, tend to be formed at the 
surface along the junction between porous and impermeable 
strata. (For a more particular account of springs, see 
Chapter XXIII.) 
(e) ludex-beds—Although it is true that the most 

