Tectonic Relief

Tectonic relief is something quite distinct from structural relief, which in the accepted usage of geomorphology is a product of erosional sculpture that has become adjusted to the structure of the region. Structural escarpments, whether at the outcrops of horizontal or inclined strata, are among its most con? spicuous features, but structural relief includes the hogbacks, homoclinal ridges, and cuestas as a whole which mark the outcrops of hard or resistant formations, with also the vales and narrower subsequent valleys picked out by long-continued denudation and river corrasion along the belts in which weaker strata outcrop. Structure, in the geological sense, is the arrangement and pattern of folding and faulting as these affect stratifled formations in particular, with also the pattern and interrelation of intrusive rock masses and the terrains which they invade; and structure is geomorphically significant when it brings weak and resistant formations into juxtaposition, so that erosion, faster on some and slower on others, is able to emphasize differences and so to shape both major and minor landforms. The forms of structural relief being entirely of erosional development, one must distinguish with care between structure and tectonics. Tectonics, i.e. the earth movements of folding and faulting which have pro? duced the pronounced structural patterns found in many landscapes, may be those of some long-past geological epoch. Long ago the relief?potentially a fantastic relief in many cases?produced by those ancient tectonics has been destroyed, for cycle after cycle, each of vast duration, may have run its course before that one started in which modern erosion has discovered and revealed the areas and belts of weaker rock, and the landscape pattern has been developed so that commonly it displays a mature relief. Purely erosional relief is familiar to British geographers, and those accustomed to the sight of the subdued land? forms of this country are apt to assume that all terrestrial relief must be of similar erosional development. Tectonic relief, on the other hand, has itself resulted primarily from earth movements; and these have taken place very recently. The major relief of peninsular Italy, for example, is of such origin. Given time, erosion will destroy it; and for this destruction only a moderate allowance of geological time is required. Compared with the intense deformations of the past, as revealed by geological evidence, those which have produced the tectonic relief existing to-day are comparatively mild: from a geological point of view, indeed, they may be thought negligible as compared with the Alpine paroxysm of orogeny which looms up in the background. But it must not be forgotten that the latter occurred millions of years ago and has been followed by an almost inconceivable amount of erosion. Milder and more modern orogenies have, moreover, been restricted to definite regions?to parts only of the mobile belts affected by Alpine orogeny. But the mobile belts as a whole occupy quite a large share of the earth's surface, and considerable parts of them are still, or have been recently, writhing. There are large regions in which, though earth movement has occurred rather recently, this has scarcely anywhere dislocated the land surface or even caused

Archaeology · Geography · Seismology · Tectonics · Geological and Geochemical Analysis · Geology · Geology and Paleoclimatology Research · Pacific and Southeast Asian Studies · Paleontology