GEOLOGY 



grows rank and abundant. Many of the common forest trees grow along the sides of the cloughs, 

 whilst the undergrowth is a tangle of the wild raspberry, bramble, honeysuckle, and ivy. Marshy 

 spots are carpeted with Sphagnum moss, whilst great clumps of bracken fern, horse-tails, and mare's- 

 tails grow in sheltered spots. The bracken fern frequently grows up above the timber line amidst 

 the grass and heather. 



The stream courses are littered along the greater part of their length with flood debris 

 of stones, and frequently it has happened that a period of flood has caused a great pile of debris 

 to accumulate in the main track of the stream, so that further progress downwards of the 

 water could only be effected by the cutting of a new passage to one side, when the stream, once 

 diverted, has continued to cut into the side of the clough until a vertical cliff has been formed, often 

 of great height. In this way a clough is sometimes seen to suddenly widen out into a sort of sylvan 

 amphitheatre, the bottom of which is filled with a level tract of bog or meadow land covered with 

 ferns and trees, and bounded by the stream, which margins on the opposite side a tall cliff 

 festooned with trailing ivy, honeysuckle, ferns, and flowering plants. 



Another special feature of these moorland cloughs is the frequency of waterfalls, owing to the 

 marked difference in hardness of the sandstones and intervening shales. 



When the stream of water in its downward course passes from a sandstone to a shale, the rate 

 of destruction of the latter is greater owing to its softness. It therefore follows that after a time 

 there is a perceptible drop in the stream level at the point where it passes from one rock to 

 the other. 



This alteration of level is naturally increased in the course of time, both by the weight of 

 water dropping from the higher level and by the wearing effect of debris brought over, until a well- 

 defined waterfall results. 



Once the waterfall is formed, it begins to be cut backwards by reason of the shale which 

 underlies the grit rock being picked out by the water of the pool formed below the fall, and 

 by spray being continually driven against it, until the outer ledge of rock over which the water 

 pours ceases to be supported from below, and it is hurled down, a new ledge or lip appearing 

 behind it. The destruction of the outer lip of the fall is accelerated by the fact that the grits are 

 usually open-jointed, and water continually finds its way down to the pool by a passage through 

 these crevices, some distance back from the edge of the fall. The passage of water through these 

 open joints results in their widening and thus allows more water to pass, the process, when long 

 continued, cutting off more or less completely the outer masses of rock until the succeeding 

 flood waters dislodge them altogether. 



Waterfalls which have arisen in this manner are common in all cloughs and add considerably 

 to their beauty. 



Where a rock is massively bedded and well jointed, the fall is broken up into irregular steps 

 formed of the various bedding planes, and the water leaps from step to step, forming miniature 

 cascades all the way. Where the sandstone is passing into a shale or where the rock of the 

 fall consists of bands of shale and grit, the face of the fall slopes outwards, and the water rushes 

 down its length like broken water down a weir. 



In some cases, a thick bed of hard grit rock overlies a still thicker bed of softer shale, and 

 where this occurs the water drops clear from a projecting ledge of sandstone into the pool below. 



The increased volume of mountain streams due to lateral feeders results in the cloughs 

 becoming widened out, and the sides are thus better exposed to the action of storms of wind 

 and rain, and frosts. As a result, they are destroyed more rapidly, and the greater part of the cliff- 

 like character is lost in the steep scree slopes already mentioned. 



The characteristics of these cloughs have been thus fully dealt with because they are one 

 of the most distinctive physical features of the moorland areas formed by the Millstone Grit, 

 and also because along their stream courses it is possible to trace the upward or downward 

 succession of the strata over great distances. 



The Millstone Grit Series everywhere underlies the productive measures, and rises into 

 moorlands on the north and east. 



As its name implies, the series consists of beds of hard quartzose grits, often very coarse, and 

 interbedded with bituminous shales and a few thin coals. In a few cases, the coals have been 

 worked to a limited extent, but they are generally much too thin to pay for working. 



The grit rocks are largely quarried for flags, building-stone, paving-stone, and road-metal. 

 The massively bedded rock bands furnish huge blocks, used as engine beds and supports for heavy 

 machinery. 



The grits contain casts of Lepidodendroid and Sigillaroid trees, not unfrequently many feet in 

 length, and two to three feet in diameter at the base. In most cases, these tree trunks have been 

 much flattened, but erect stumps, still circular and 6 to zoo feet in height, are found, as at Oldham 

 Edge, with the marks of the leaf-bases clearly impressed upon them. 



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