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veals thin layers of Sandstone, at first nearly horizontal, suddenly flung back upon each other, and helping to form great
In the Thurstaston and Wallasey Sections, again, not only are the layers seen to have assumed a vertical position at some points, but they further shew a minute system of dislocations. The alternation of bands of large well-rounded grains of sand with bands of finer material has frequently resulted in the more rapid weathering of the former; so that little ridges of Sandstone protrude, and offer a lodgment to atmospheric dust and vegetable mould. Surface markings of this character, added to the chisel-marks of the roadmaker, cause considerable difficulty to be experienced in examining the artificial sections revealed at Wallasey and Bidston.
It appears, then, from the foregoing, that there is something more than a superficial appearance to account for. We must strive to explain why certain bands of Sandstone forming part of the Bunter series should appear as if, on a small scale, they had been violently contorted and dislocated. That the material was not accumulated in this form is evident when the reversed folds, the minute puckerings, and the dislocations are considered.
In commencing the enquiry it is possible to reduce the question to a comparatively narrow limit by deciding when the contortions occurred. At what period in the history of the Sandstones were these structural characters impressed upon them ? was it while the layers of sand were being accumulated on the floor of the Triassic sea ? or was it at a later period when the thick masses of Bunter Sandstone were undergoing consolidation ? If not at either of these periods . then it must have been subsequent to consolidation. So far as the sections now under consideration are concerned there is evidence to shew that the events occurred in neither of the two stages last mentioned. The joints, which mark the time of consolidation, are seen traversing the rocks as straight lines even when cutting across the areas where contortion is most marked. Any appeal to those internal forces which are
the acknowledged source of contortions affecting rocks on a large scale is out of the question here, as the bands affected are seen to be overlaid by others shewing no signs of contor
The evidence seems, then, to point to a contemporaneous origin. The sandy layers must have been contorted as they formed the floor of the Triassic sea, and before they were covered up by the sheets now resting upon them.
The agencies hitherto recognised as capable of producing contemporaneous contortions are not numerous. Drifting masses of ice, which have been a fruitful source of disturbance in Post Tertiary Sands, will not avail us in the present case. None of the Bunter contortions are associated with a shelving beach on which such masses could have been stranded. Nor do I know of any instance in which ice-borne fragments of rock, the almost inevitable accompaniment of floating bergs, have been found included in the Wirral Sandstones. On the Southern and Eastern coasts of England, in the present age, contortions must frequently be produced where tracts of land slide bodily into the sea. But there is no reason to suspect that any elevated land masses existed in the immediate neighbourhood of any of the sections now under consideration.
Let us examine more particularly the surroundings of our sections. It will be seen that denudation in one case, and denudation combined with faulting in the other, have removed some of the strata which were originally continuous with, or resting upon, the contorted sheets at Thurstaston and Bidston; so that in these two sections we are left without much information that might have been found useful in the present enquiry. The following points may, however, be noticed as occurring at Thurstaston in the section exposed on the Southern face of the rugged outlier known as • Thor's Stone." In the South-Eastern corner laminæ having a vertical arrangement are suddenly cut short above, and overlaid by laminæ of Sandstone having at first a normal arrangement, but becoming contorted higher up. In the upper portion of the Stone generally, the
lamina shew current bedding. A singular appearance is noticeable in the South-Western corner. Current bedded laminæ, rising gently to the East, are seen to have been violently flung back upon themselves, a minute system of dislocations being developed at the same time. To the right of this highly disturbed area the lamination is undistinguishable, and the arrangement may be described as being chaotic. In the cutting at Bidston, too, there is present a system of curves and flexures, accompanied by minute puckerings and slight dislocation, the uppermost part of the section showing a normal bedding. The section exposed by the roadside in Wallasey Village is more complete than either of the others. Both to the left and right of the contorted bands current-bedding is developed at a smarter angle than usual; while the whole series is overlaid by sheets which are nearly horizontal. To the right the contortions have not died away at the point where contemporary denudation has stripped away a part of the original floor, but the section is more complete on the opposite side of the disturbed area.
And now as to the origin of these strange appearances. Professor Geikie, who has treated of this matter in his recently-published “Text Book of Geology," states (p. 479)
• ” that “curved and contorted lamination is of frequent occurence among Palæozoic Sandstones,” but adds that the cause of this structure is not well understood.” One of the illustrations given by him is taken from the Cambrian Sandstones of Gairloch. It will be seen that in the Wallasey and Thurstasten sections current bedding is constantly associated with contortion. An enquiry into the conditions under which current bedding was developed and contemporary denudation carried on at the time when the Bunter Sandstones of Wirral were being accumulated, may help to explain the contorted structure; the probability being that the same forces which produced the former appearances, were indirectly the cause of the latter.
The New Red Sandstone of England is regarded as having been laid down in a large inland sea, or series of lakes.
Into these basins, large rivers would discharge their loads of gravel, sand and clay. The disposition of this sediment on the floor of an inland sea would be far different from that found to prevail where rivers similarly charged have entered oceanic basins. In the latter case there is usually a powerful current, either tidal or oceanic, sweeping past the estuary or delta, so that the river frequently maintains itself as a stream, far out into the sea. In this way a complete arrangement of materials according to weight would tend to follow, coarse gravel and sand being the first to settle down, while the finer sand and mud would cover extensive tracts of the sea-bottom, at a greater distance from land. In an inland sea, on the other hand, rivers would have their velocity checked very speedily, so that a greater portion of the sediment would be deposited close to the shore. The rivers, too, would split up into an indefinite number of currents, each traversing the waters of the sea for a short distance, with a rapidly decreasing speed. From extended observations of the disposition of the sediment composing the Bunter series of Wirral, I am inclined to think that, in one respect, material suspended in these currents was afterwards deposited in a manner similar to that found prevailing at the mouth of the Mississippi to-day. Here the snbmarine channels are so continuously built up by deposition of sediment on their banks and floors that they at last begin to appear above the surface of the water. The “ dip” of the oblique lamination developed under such circumstances would be directed towards the centre of the channel. In the New Red Sandstone channels have been built up in a similar manner, as shewn in the Middle Eye section. See figure.) There are also constant indications of branchings or deviations from old courses. Denudation of sediment already laid down would be the result of such movements. Yet, even this will not explain all the phenomena connected with current-bedding as developed in the New Red. I think we require to apply Professor Ramsay's well-known diagram illustrating the behaviour of uinding rivers to the water
courses of the Triassic Sea. The Middle Eye section shews this very well. A band of hard conglomeratio sandstone, resting on the red conglomerate which forms the base of the cliff at this point, was partially denuded from the South. Erosion appears to have taken place rapidly on the Northern bank of the channel, whilst oblique lamina were being deposited on the more sheltered Southern bank. This was probably of far greater height than the present upward termination of the laminæ and possessed considerable steepness. Higher up in the section are to be seen the layers accumulated on the shelving bank of another channel. The superior thickness of this bed shews that it escaped denudation to a greater degree than the lower
Having now recognised the existence of powerful denuding currents and steep sandbanks in the Triassic Sea, we may refer again to the Wallasey section shewing contortion. At the Southern end we note that disturbed sheets are brought to an abrupt termination at the edge of what was once a tall sandbank. Upon this steep slope oblique layers of sand were afterwards laid down. Northwards, a great thickness of material, in section not deviating very much from the horizontal, is brought to an abrupt ending in much the same manner. A long sandbank, facing South, marks the Northern limits of a denuded area. On the steep side of this bank the current afterwards laid down sheets of sand having a somewhat perilous slope towards the channel. May it not be that in the instability resulting from such conditions of accumulation we have a key to the origin of the contortions close by. The upper portions of a steep bank, in gravitating to a lower level, would most probably do so at the expense of the sheets then forming the floor of the channel. Subjected to great lateral pressure, these would be thrown into a series of curves and folds, and might possibly be faulted where the pressure was unusually severe or too suddenly applied.
It is to be remarked that the Northernmost beds in the