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 débris of stones, and frequently it has happened that a period of flood has caused a great pile of débris 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 100 feet in height, are found, as at Oldham Edge, with the marks of the leaf-bases clearly impressed upon them.
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