
STRUCTURE AND SURFACE FEATURES 411 
2 PLAINS AND. PLATEAUS 
In discussing the structure and origin of mountains, some 
reference has been made to the formation of plains and 
plateaus. It may be well, however, to summarise here the 
general characters of those particular land-forms. 
(a) Plains.—These may be defined as areas of approxi- 
mately flat or gently undulating land. They are usually 
confined to lowlands, but in the case of very extensive areas 
the surface of a plain may rise by almost imperceptible 
degrees to a height of several thousand feet. This, however, 
is exceptional, such elevated tracts being usually termed 
plateaus. 
Plains of Accumulation.—These are built up of horizontal 
strata, so that the surface is a more or less exact expression 
of the internal geological structure. Plains of this type have 
been formed in various ways. Many are of lacustrine, 
fluviatile, or estuarine origin; in other words, they consist 
of undisturbed aqueous deposits. Others, again, such as 
many coastal plains, have been formed partly by aqueous 
- sedimentation, and partly by wind blowing sand before it 
from exposed beaches. When a plain occurs at or near a 
base-level of erosion, rain and running water have little 
effect upon it—the process of denudation is practically at a 
standstill. Under certain conditions, however, the surface 
may be considerably modified by the action of wind. For 
example, deltas and coastal plains margined by the sea or 
by an extensive lake, are not infrequently invaded by sand 
_dunes. The higher a plain rises above its base-level, the 
more does it become subject to denudation—high-lying plains 
usually showing a more irregular and undulating surface 
than those occurring at lower levels. It must be noted, 
however, that the form of the surface depends to a large 
extent upon the nature of the materials of which the plain 
is composed. Other things being equal, a plain consisting 
chiefly of impervious deposits is more readily eroded than 
one built up largely of gravel and sand and other more or 
less porous accumulations. As examples of plains of 
accumulation may be mentioned the fluviatile plains and 
deltas of the Nile, the Danube, the Ganges, the Amazon, 

