
STRUCTURE AND SURFACE FEATURES 413 
Colorado, the Uplands of Abyssinia, and the Deccan of India 
—all more or less highly denuded regions. 
Plateaus of Evroston—The general surface of these 
plateaus having been determined by denudation, it rarely 
Or never coincides with the geological structure. But the 
structure and origin of erosion plateaus have already been 
sufficiently discussed. 
3. VALLEYS 
By the term valley, we usually mean the hollow or 
depression through which a stream or river flows. Some 
valleys, however, contain no streams, but are mere elongated 
depressions. With relatively few exceptions, valleys are 
either (a) the direct result of erosion or (4) have been greatly 
modified by it. If we consider the latter, however, from the 
point of view of their origin, they must be distinguished from 
true erosion valleys—just as tectonic mountains must be 
recognised as such, even although they have all been more 
or less modified by denudation. We can therefore group 
valleys in two classes:-—(1) Valleys which owe their origin 
either to hypogene action or to epigene action other than 
that of erosion; and (2) valleys which are true hollows of 
erosion. 
I. ORIGINAL OR TECTONIC VALLEYS.—These are of two 
kinds—(a) elongated hollows produced by the irregular 
accumulation or heaping-up of materials at the surface; and 
(4) depressions which are the result of crustal deformation. 
(2) Constructional Valleys.—This class of valley is of 
little importance. It is represented in volcanic regions by 
depressions occurring in the surface of various volcanic 
accumulations, and by the now and again pronounced 
hollows that separate adjacent cones, lava-flows, or heaps 
of ejecta. Similarly, the depressions lying between range 
of dunes and moraines may be termed constructional valleys. 
In a word, any hollow at the surface caused by the irregular 
distribution of materials, whether by volcanic action or by 
epigene action of any kind, would come under this head. 
(6) Deformation Valleys.—Theoretically, we may group 
these as (1) Déslocation Valleys, and (2) Synclinal Valleys. 
Not infrequently, however, a deformation valley has been 
