


308 STRUCTURAL AND FIELD GEOLOGY 
These old sedimentary formations, since the time of their 
elevation, have been subjected to very considerable erosion, 
but, owing to their generally unconsolidated character they 
are not distinguished by any very prominent surface features 
—but form, for the most part, gently undulating low grounds 
and plains.* The mapping of such accumulations is attended 
with some difficulty—it being often hard to trace the outcrops. 
This is due, in the first place, to the fact that upon slopes the 
junction-lines are obscured by the washing down of materials 
from above—the outcrops of lower beds being often entirely 
concealed under sand, loam, etc., derived from overlying strata. 
Geologists mapping in such regions occasionally employ a 
gouge-like spud, which might be described as a kind of 
exaggerated “cheese-taster,” for the purpose of ascertaining 
the position of the concealed outcrops as accurately as 
possible. The annexed diagram will serve to illustrate 
the modus operandi (Fig. 120). The surface from x to 6 shows 

FIG. 120,—CONCEALMENT OF OUTCROP BY SURFACE WASH. 
Clay (a) overlaid by sand (0). 
nothing but sand, we shall suppose, while between a and x 
clay obviously immediately underlies the soil. The observer 
having reason to believe that the sand at x and for some 
* The Tertiary deposits which in England and the low grounds of 
Middle Europe generally are usually unconsolidated and not much 
disturbed—spreading as sheets of greater or less thickness over Mesozoic 
and older rocks—are represented in Southern Europe by much more 
massive strata—the older portions of which enter largely into the frame- 
work of the Pyrenees, the Alps, the Apennines, etc. It would be an 
abuse of terms to speak of these deposits as ‘“‘superficial formations.” 
Even in this country, where the corresponding deposits are of slight 
thickness and more or less unconsolidated, they are, nevertheless, not 
included by geologists amongst “superficial formations”—this term 
being restricted to post-Tertiary and recent accumulations alone. From 
the point of view, however, of the field geologist, all loose and uncon- 
solidated beds of gravel, sand, clay, loam, etc., may be looked upon as 
superficial accumulations. 

