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270 or even 300. In many places they are yet steeper, their upper portions especially becoming quite precipitous. They everywhere exhibit a well-marked terraced character; precipices and long walls of bare rock rise one above another, like the tiers of some cyclopean masonry, and are separated usually by short intervening slopes, sparsely clothed with grass and moss, or sprinkled with tumbled rock-rubbish. The coasts are usually precipitous, many of the islands having only a few places where a landing can be effected. Not a few are girt by cliffs, ranging in height from 200 or 300 feet up to 1000 feet, and even in some places exceeding 2000 feet. The best-defined valleys are broad in proportion to their length. Followed up from the head of a sea-loch, they rise sometimes with a gentle slope until in the distance of two or three miles they terminate in a broad amphitheatrelike cirque. In many cases, however, they ascend to the water-parting in successive broad steps or terraces. Each terrace is cirque-shaped, and framed in by a wall of rock, the upper surface of which stretches back to form the next cirque-like terrace, and so on in succession until the series abruptly terminates at the base, it may be, of some precipitous mountain. Occasionally the neck between two valleys running in opposite directions is so low and flat that it is with difficulty that the actual water-parting can be fixed. In such cases we have a well-defined hollow, bounded by precipitous, terraced hill-slopes, crossing an island from shore to shore. Were the land to be slightly depressed such hollows would form sounds separating adjacent islands, while the valleys that head in cirques would form sea-lochs. There can be no doubt, indeed, that the existing fiords of the Faroes simply occupy the lower reaches of land-valleys, and that the sounds separating the various islands from each other in like manner indicate the sites of long hollows of the character just described. In a word, the islands are the relics of a plateau of comparatively recent geological age, for the rocks date no further back than Oligocene times. All the land-features are the result of subaerial erosion guided and determined by the petrological character and horizontal arrangement of the strata. The precipitous cliffs of the coast-line owe their origin, of course, to the undermining action of the sea, the rocks ever and anon giving way along the well-marked vertical joint-planes.

In Great Britain horizontal strata occupy no broad areas. But wherever they put in an appearance they yield the same surface-features. Thus in the north-west Highlands we have the striking pyramidal mountains of Canisp, Suilven, and Coulmore, carved out of horizontal red sandstones of Pre-Cambrian age. In Caithness, again, we have the peaked and truncated pyramids of Morven, Maiden Pap, and Smean, hewn out of approximately horizontal Old Red Sandstone strata. Ingleborough is another good example of a pyramidal mountain having a similar geological structure. Many illustrations are likewise furnished by the horizontal strata of other lands. Thus pyramidal and more or less abrupt hills, the precipitous sides of which are defined by vertical joints, are common in the horizontally bedded " Quadersandstein" of Saxon Switzerland. So again in the region of the Dolomites, whenever the strata are horizontal the mountains carved out of them tend to assume pyramidal forms. In a word, we may say that all the world over the same geological structure gives rise to the same land-forms.

River-courses hewn in horizontal strata will vary somewhat in form according to the nature of the rocks and the character of the climate. In regions built up of relatively unyielding rocks, or of alternations of these and less resisting beds, the valleys tend to be trench-like, and the mountain-slopes are more or less abrupt. But under the influence of rain, springs, and frost these harsh features are toned down, river-cliffs are benched back, and abrupt acclivities are replaced by gentler slopes. Should the strata consist of soft materials throughout, there will be a general absence of harsh features; round-topped hills and moderate valley-slopes will characterise the land. Nevertheless, whether the strata be "hard " or "soft," thick-bedded or thin-bedded, or show alternations of many different kinds, and whether the climate be arid or humid, equable or the reverse—tropical, temperate, or arctic—the same general type of surfacefeatures can always be recognised.

CHAPTER IV

LAND-FORMS IN REGIONS OF GENTL Y INCLINED

STRA TA

ESCARPMENTS AND DIP-SLOPES—DIP-VALLEYS AND STRIKE-VALLEYS—FORMS ASSUMED BY A PLATEAU OF EROSION—VARIOUS DIRECTIONS OF ESCARPMENTS—SYNCLINAL HILLS AND ANTICLINAL HOLLOWS—ANTICLINAL HILLS.

THE most characteristic land-forms met with in regions where the strata are inclined in some general direction are escarpments and dip-slopes, the former coinciding with the outcrops, and the latter with the inclination or dip of the strata. In such regions some streams and rivers not infrequently flow in the direction of dip, and thus cut across the escarpments, while others may traverse the land along the base of the escarpments.

The origin of these phenomena is not hard to trace. Let us suppose that some wide tract of horizontal strata has been elevated along an axis so as to form a considerable island. If the movement of elevation were slowly effected the sea would doubtless modify the land-surface as it arose, but for simplicity■s sake we shall ignore such action, and suppose that the new-born land exists as an elongated island, the surface sloping away at a low angle on either side of a somewhat flattened axis. (Fig. 12.) At first, then, the surface coincides with the underground structure —a dome-shaped land formed of dome-shaped strata. (Fig. 13.) It is obvious that the drainage will be in

[graphic]

Fig. 12. Map Of As Island Composed Of Dome-shaped Strata.

The strata an inclined is the direction of the arrows.

the direction of the dip of the strata—all the main rivers will take the quickest route to the sea. But as we cannot suppose that the surface of the new-made land would be without some irregularities, the streams and rivers would not actually follow straight courses.

[graphic]

Fig. 13. Section Through The Island Shown In Fig. 12.

Slopes of surface coincide with arrangement of strata.

On the contrary, it could not but happen that one stream would eventually join another, and in this way many might become tributaries of one or more large rivers. Thus we should have certain courses cut in the general direction of the dip, while others joining these would in some places go with the inclination of

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