These valleys are not synclinal foldings of rocks; they seem to be erosion-valleys.
These bands are often concealed by more recent deposits, but it is clear that in this region the Devonian beds form a basin or synclinal with the Amazon for its axis.
In the basin of the Lower Amazon the Carboniferous beds lie within the Devonian synclinal and crop out on both sides of the river next to the Devonian bands.
The western half of the basin occupies a trough of synclinal structure; but the making of this syndine is so ancient that it cannot be directly connected with the occurrence of the lake to-day.
A strong synclinal fold traverses Eday and Shapinsay, the axis being N.
Clew Bay, with its islets capped by glacial drift, is a submerged part of a synclinal of Carboniferous strata, and Old Red Sandstone comes out on the north side of this, from near Achill to Lough Conn.
Sediments approaching to the culm type, with similar flora and fauna, were deposited in synclinal hollows in parts of France and Spain.
This basin may be, on the whole, considered as a continuation of the synclinal depression of the Manych, which stretches along the northern foot of the Caucasus from the Sea of Azov.
Along the southern margin of the Triassic area there is a long band of Jurassic beds dipping towards the Danube; and at its eastern extremity this band is continuous with a synclinal of Jurassic beds, running parallel to the western border of the Bohemian massif, but separated from it by a narrow strip of Triassic beds.
North of the main water-parting of Afghanistan the broad synclinal plateau into which the Hindu Kush is merged is traversed by the gorges of the Saighan, Bamian and Kamard tributaries of the Kunduz, and farther to the west by the Band-i-Amir or Balkh river.
Here there intervenes a second wide synclinal plateau, of which the northern edge is defined by the flat outlines of the Elburz to the south of Mazar-i-Sharif, and immediately at the foot of this range lie the alluvial plains of Mazar and Tashkurghan.
The Cretaceous beds form a synclinal upon the eastern side of the chain (and, in general, beyond the Chilean boundary), while the Jurassic beds are thrown into a number of folds which form the axis and the western flank.
At some period subsequent to this deposition there was a movement of elevation, which appears to have thrown the whole mass of rocks into a fold along an anticlinal axis running west and east, which was flanked to north and south by synclinal hollows.
The great coal-field on the south is a perfect example of a synclinal basin, the Millstone Grit and Carboniferous Limestone which underlie the Coal Measures appearing all round the margin.
The peninsula of Cornwall and Devon may be looked upon as formed from a synclinal trough of Devonian rocks, which appear as plateaus on the north and south, while the centre is occupied by Lower Carboniferous strata at a lower level.
The positions of springs are determined by permeable depressions in the surface of the ground below the general level of saturation, and frequently also by the holding up of that level locally by comparatively impermeable strata, sometimes combined with a fault or a synclinal fold of the strata, forming the more permeable portion into an underground basin or channel lying within comparatively impermeable boundaries.
- Plan of Anticlinal and Synclinal Folds.
On the other hand, in advancing southwards along the synclinal axis, he loses stratum after stratum and gets into lower portions of the series.
The Carboniferous Limestone, laid down in a sea which covered nearly the whole Irish area, appears in the synclinal folds at Cork city and Kenmare, and is the prevalent rock from the north side of the Knockmealdown Mountains to Enniskillen and Donegal Bay.
The only important occurrences of coal in the south are in eastern Tipperary, near Killenaule, and in the Leinster coalfield (counties Kilkenny and Carlow and Queen's County), where there is a high synclinal field, including Lower and Middle Coal-Measures, and resembling in structure the Forest of Dean area in England.
The present central plain itself may be regarded as a vast shallow synclinal, including a multitude of smaller folds.
The earth-wrinkles of this epoch were turned into a north-easterly direction by the pre-existing Leinster Chain, and the trend of the anticlinal from Limerick to the Slieve' Bloom Mountains, and that of the synclinal of Millstone Grit and CoalMeasures from Cashel through the Leinster coalfield, bear witness to the resistance of this granite mass.
Towards the extreme west and south, anticlinal and synclinal ridges trend north and south, the most characteristic being the Huxley, Owen, Sedgwick, Franklin and Arthur Ranges.
The Cretaceous beds lie in a broad synclinal upon the eastern flank, but the greater part of the chain is formed of Jurassic beds, through which, on the western margin, rise the numerous andesitic volcanic centres.