

STRUCTURE AND SURFACE FEATURES 407 
the Rhenish Schiefergebirge, the Scandinavian Mountains are 
all examples of highly denuded plateaus of erosion. 
At a very early geological period, lofty ranges of Tectonic 
mountains extended over what are now our Northern High- 
lands and Southern Uplands. During prolonged ages those 
ancient Caledonian ranges were subject to erosion, until 
eventually they were largely reduced to their base-level—only 
a few sorely wasted stumps and cores projecting above a 
gently undulating plain of erosion. Subsequently depression 
ensued, and the plain of erosion became, over considerable 
areas, more or less deeply buried under sedimentary deposits. 
To trace the geological history in detail is here impossible— 
it is too long a tale to tell—and we need do no more than 
realise the fact that eventually all the SESS areas were 
again re-elevated ex masse. 
The Highlands and Southern Uplands then appeared as 
plateaus. Their configuration was upon the whole plain-like, 
the peripheral areas being to some extent occupied with 
approximately horizontal sedimentary strata, resting upon 
and concealing the old plain of erosion. In the central and 
more elevated portions of the plateaus that old plain formed 
the surface, and appears to have been here and there 
diversified by more or less abrupt heights—the worn and 
abraded torsos of the ancient Caledonian Mountains. 
In the course of long ages the plateaus in question have 
experienced excessive denudation. To such a degree, indeed, 
have-they been trenched and furrowed by multitudinous valleys, 
that they are now hardly recognisable as tablelands; their 
original plain-like character is well-nigh lost. They have 
been converted into rolling uplands, into regular ranges or 
irregular groups and masses of Relict mountains, the con- 
figuration and distribution of which have been determined 
very largely by the nature of the constituent rocks and the 
mode of their arrangement. 
Sometimes, as throughout the larger portion of the 
Highlands and Southern Uplands, the Relict mountains have 
been sculptured out of the highly folded rocks forming the 
old plain of erosion; in other places, they are simply remain- 
ing portions of the younger rocks which overlie that plain ; 
while in not a few cases, the upper part of a mountain 
