Denudation, consequent on the renewed uplift of the country, affected the Jurassic beds until the middle of Cretaceous times.
Along the coastal margins they underlie the newer formations and appear in the deep valleys and kloofs wherever denudation has laid them bare.
It is, however, still possible to distinguish some traces of this formation towards the east, where atmospheric denudation has been less active.
The soft shales or clays of the hills bounding the valley render these hills especially subject to the action of denudation, and the result, in rounded slopes and easily accessible crests, determines the nature of the easy tracks and passes which intersect them.
In width, called the Dasht-i-Hamdamao, or Dasht-i-Ardewan, formed by the talus or drift of the higher mountains, which, washed down through centuries of denudation, now forms long sweeping spurs of gravel and sand, scantily clothed with wormwood scrub and almost destitute of water.
Of these the most notable is the Niagara escarpment which extends eastward from Canada, past Lewiston and Lockport, - a downward step from the Erie to the Ontario plain, where the Niagara limestone outcrops, and its resistance to denudation accounts for the steeply rising face at the boundary between the two plains.
As a rule the crests of the ranges are worn down by aerial denudation and have the general appearance of rounded domes.
Denudation in earliest Eocene times has produced flint gravels above the chalk, and an ancient stream deposit of chalk pebbles occurs at Ballycastle.
As is always the case in the broad denudation of the gently inclined strata of such plains, the weaker layers are worn down in sub-parallel belts of lower land between the oldiand and the belts of more resistant strata, which rise in uplands.
Provpee The province was uplifted and divided into great blocks by faults or monoclinal flexures and thus exposed to long-lasting denudation in a mid-Tertiary cycle of erosion; and then broadly elevated again, with renewed movement on some of the fault lines; thus was introduced in late Tertiary time the current cycle of erosion in which the deep canyons of the region have been trenched.
These strata have been subjected to great denudation, but owing to their comparatively soft character this has been, in the main, nearly uniform, and has produced no very bold features of relief.
That those to the westward have long been inactive is shown by the destruction of craters by denudation, by deep ravines, valleys and tall cliffs eroded on the mountain sides, especially on the windward side, by the depth of soil formed from the disintegrated rocks, and by the amount as well as variety of vegetable life.
The rate of denudation in exposed positions is exceedingly rapid; while spots sheltered from the sand blast suffer a minimum of erosion, as shown by the preservation of ancient inscriptions.
The mountains in fact have been hewn out of the original bulk of the land in proportion as the valleys have been excavated_ The denudation would continue so long as the ground stood above the level of the sea; but there have been prolonged periods of depression, when the ground, instead of being eroded, lay below the sea-level and was buried sometimes under thousands of feet of accumulated sediment, which completely filled up and obliterated the previous drainage-lines.
(For the chief rivers see the separate articles on them, and also the section on the physical features in the article on the different shires of Scotland.) The topography of the country being the result of prolonged denudation, it is reasonable to infer that the oldest surfaces likely to be preserved are portions of some of the platforms of erosion successively established by the wearing down of the land to the sea-level.
Though dating back only to older Tertiary time, this plain has been so deeply trenched by the forces of denudation that it has been reduced to mere scattered fragments.
Each ridge and mountain has been cut into its shape by denudation, but its outlines have been determined by the nature of the rocks and the manner in which they have yielded to decay.
The most impressive form of solitary cone is that wherein after vast denudation a thick overlying formation has been reduced to a single outlier, such as Morven in Caithness, the two Bens Griam in Sutherland, and still more strikingly, the pyramids of red sandstone on the western margin of the shires of Sutherland and Ross and Cromarty.
That the volcanic period was a prolonged one is shown by the great denudation of the plateaus before the last eruptions took place.
Denudation has continued active ever since, and now, owing to greater hardness and consequent power of resistance, the glassy lava stands up as the prominent and picturesque ridge of the Scuir, while the basalts which formerly rose high above it have been worn down into terraced declivities that slope away from it to the sea.
It lies in a basin among granite hills, nowhere exceeding 2627 ft., remarkable for their denudation and their abrupt black crags and pinnacles.
At the close of the pre-Cambrian, many of the deposits of that period must have been elevated into regions of fairly high ground; this we may assume from the nature of the Cambrian deposits which are mainly the product of the denudation of such ground.
On the plateau itself the sandstone is generally overlaid by the Deccan trap, a blackish-coloured basaltic rock of volcanic origin, the high level tableland having been formed by a succession of lava flows, the valleys of Central India being merely "denudation hollows" carved out by the action of rain and rivers.
The denudation and destruction of the granites gave rise to the Ladoga schists and various deposits of the same period, which were subsequently strongly folded.
New masses of granites protruded next from underneath, and the Bothnian deposits underwent foldings in their turn, while denudation was again at work on a grand scale.
Its surface is a succession of great terraces, facing north and north-east, formed by the denudation of the ancient sandstone plateau which once covered this part of the continent; the terraces are seamed by watercourses, and their valleys are broken by hills and ranges of highlands.
As they pass away southwards this gridiron formation strikes with a gentle curve westwards, the narrow enclosed valleys widening out towards the sources of the rivers, where ages of denudation have worn down the folds and filled up the hollows with fruitful soil, until at last they touch the central waterdivide, the key of the whole system, on the Quetta plateau.
Being fed by tributaries which for the most part drain narrow valleys where gradual denudation has washed bare the flat-backed slopes of limestone ridges, and which consequently send down torrents of rapidly accumulating rainfall, both these central lines of water-course are liable to terrific floods.
Possibly the ordinary processes of denudation and erosion, acting on those recent deposits which overlie the harder beds of the older series, may have much to say to these climatic changes, and the wanton destruction of forests may have assisted the efforts of nature; but it is difficult to understand the widespread desiccation of large areas of the Baluch highlands, where evidences of Arab irrigation works and of cultivation still attest to a once flourishing agricultural condition, without appealing to more rapidly destructive principles for the change.
North of Bhutan, between the Himalayan crest and Lhasa, this formation is approximately maintained; farther east, although the same natural forces first resulted in the same effect of successive folds of the earth's crust, forming extensive curves of ridge and furrow, the abundant rainfall and the totally distinct climatic conditions which govern the processes of denudation subsequently led to the erosion of deeper valleys enclosed between forest-covered ranges which rise steeply from the river banks.
They are all similar in the great features of their land-forms, which have been impressed upon them by the prolonged action of atmospheric denudation rather than by the original order and arrangement of the rocks; but each group has its own geological character, which has imparted something of a distinctive individuality to the scenery.
J?-,, r r, , // Saurian to Crystalline Rocks Permian & Carboniferous ® Igneous Rocks Devonian 1 Jurassic Trsasiic have been described; but denudation has been carried further than in the western Alps, and accordingly the masses overlying the thrust-planes have been more completely removed (q.v.).
Excavations carried on in 1910 showed that the town of the "Mycenean" period which lay on the left bank of the Eurotas a little to the south-east of Sparta was roughly triangular in shape, with its apex towards the north: its area is approximately equal to that of Sparta, but denudation and destruction have wrought havoc with its buildings and nothing is left save ruined foundations and broken potsherds.
It appears to approximate in time to the similar earth movement and denudation at the close of the palaeozoic period in Europe.
The Upper Coal-Measures, as a rule, have been lost by denudation, much of which occurred before Triassic times.
The Triassic beds rest on the various Carboniferous series in turn, indicating, as in England, the amount of denudation that followed on the uplift of the Hercynian' land.
The Senonian chalk, or " White Limestone," is hard, with numerous bands of flint, and suffered from denudation in early Eocene times.
The volcanic plateaus clearly at one time extended far west and south of their present limits, and the denudation of the lava-flows has allowed a large area of Mesozoic strata also to disappear.
Thereafter, in papers published by the Cambridge Philosophical Society and the Geological Society of London, he entered largely into mathematical inquiries connected with geology, dealing with the effects which an elevatory force acting from below would produce on a portion of the earth's crust, in fissures, faults, &c. In this way he discussed the elevation and denudation of the Lake district, the Wealden area, and the Bas Boulonnais.
The particular bed marked EF has been entirely removed by denudation from the top of the anticline, and is buried deep beneath the centre of the syncline.
The portion of an anticline which has been removed by denudation is the "aerial arch," dotted in fig.
For example, in many coal regions the deposits have been conserved in some districts in the synclines or "basins," while they have been removed by denudation from the uplifted anticlines in others.