Arrangement and Direction of Parallel Folds of Strata.—The possible causes of the folding of strata by lateral movements have been considered in a former part of this chapter. No European chain of mountains affords so remarkable an illustration of the persistency of such flexures for a great distance as the Appalachians before alluded to, and none has been studied and described by many good observers with more accuracy. The chain extends from north to south, or rather N.N.E. to S.S.W., for nearly 1500 miles, with a breadth of 50 miles, throughout which the Palæozoic strata have been so bent as to form a series of parallel anticlinal and synclinal ridges and troughs, comprising usually three or four principal and many smaller plications, some of them forming broad and gentle arches, others narrower and steeper ones, while some, where the bending has been greatest, have the position of their beds inverted, as before shown in Fig. 73, p. 87. The strike of the parallel ridges, after continuing in a straight line for many hundred miles, is then found to vary for a more limited distance as much as 30°, the folds wheeling round together in the new direction and continuing to be parallel, as if they had all obeyed the same movement. The date of the movements by which the great flexures were brought about must, of course, be subsequent to the formation of the uppermost part of the coal or the newest of the bent rocks, but the disturbance must have ceased before the Triassic strata were deposited on the denuded edges of the folded beds. The manner in which the numerous parallel folds, all simultaneously formed, assume a new direction common to the whole of them, and sometimes varying at an angle of 30° from the normal strike of the chain, shows what deviation from an otherwise uniform strike of the beds may be experienced when the geographical area through which they are traced is on so vast a scale. The disturbances in the case here adverted to occurred between the Carboniferous period and that of the Trias, and this interval is so vast that they may have occupied a great lapse of time, during which their parallelism was always preserved. But, as a rule, wherever after a long geological interval the recurrence of lateral movements gives rise to a new set of folds, the strike of these last is different. Thus, for example, Mr. Hull has pointed out that three principal lines of disturbance, all later than the Carboniferous period, have affected the stratified rocks of Lancashire. The first of these, having an E.N.E. direction, took place at the close of the Carboniferous period. The next, running north and south, at the close of the Permian, and the third, having a N.N.W. direction, at the close of the Jurassic period.[14] Fig. 78: Unconformable junction of old red sandstone and Silurian schist at the Siccar Point, near St. Abb’s Head, Berwickshire.