GEOLOGY
Lane, whilst on the north side of the latter is a quarry showing the intervening shales. It is in the higher bed of limestone that crinoid stems occur in greatest abundance, the upper 40 feet at Salt Hill being almost entirely made up of them. The same, or a similar bed, is seen at Whitewell. Many of the rough field walls are built of this rock, which readily breaks up, the crinoid stems weathering out in high relief.
Both the lower and upper beds are much quarried for lime-burning, that derived from the black limestone being especially good.
PENDLESIDE GROUP
This group, as its name implies, occurs on the flanks of Pendle Hill, of which it forms what have been called the buttresses of the north-western slope. This slope rises to a height of 1,831 feet, and shows a regular succession of deposits from the Carboniferous Limestone to the Pendle Grit. The stream courses from the summit have cut down through the beds, so that it is possible to work out in them the full succession, and Dr. Wheelton Hind and Mr. J. Allen Howe have determined the sequence as follows[1]:—
Pendle Grit, or 'Upper Yoredale Grit.'
Bolland Shales, including the 'Lower Yoredale Grit,' or Pendleside Grit.
Pendleside Limestone with overlying Shales and Mudstones.
Black Shales with a few bands of impure Limestones.
Shales with Limestones.—These beds consist of shales, thin limestones, mudstones, and at times thin ironstone. They are exposed in the Pendle branch of the Worston Brook by the lane east of Worston, and the brooks flowing from Lower Gills to Ings Beck near Skeleron Mines.[2] The upper beds consist of limestone from one to three feet in thickness, which regularly alternate with clayey shale.
In brook courses, as at Angram-Green near Worston, the rocks form a series of waterfalls, owing to the markedly unequal erosive action of the streams upon the clay-shale and limestones.
The Geological Survey calculated the thickness of this division as close upon 2,500 feet thick, but the estimate is considered too high by Dr. Hind and Mr. Howe, who calculated it at 1,500 feet.[3] Many of the springs issuing from these shales are charged with sulphuretted hydrogen.
The Pendleside Limestone has a thickness estimated by Professor Hull at 350 feet, and consists of a series of thin limestones and shales below, passing into thicker beds of limestone and a few shales above, the upper member being a bed of large hard 'bullions,' which contain a goniatite, Glyphioceras reticulatum.
The upper limestones contain crinoid stems and examples of Productus scabriculus and P. semireticulatus, forms which pass up into the Millstone Grit and Lower Coal Measures. The black shales at the base contain species of Chonetes, Productus, Prolecanites, and Orthoceras.
The series is also developed around the flanks of Longridge Fell, where it contains well-bedded dark limestones and shales. Sections can be seen in a quarry north of the Longridge and Clitheroe road, three-quarters of a mile east of Thornley Hall.[4]
At Black Hall and Cold Coats quarries, the lower beds are fairly fossiliferous, numerous species of goniatites being found, together with Posidoniella lævis and Posidonomya Becheri.
Bolland (Bowland) Shales, with the 'Lower Yoredale Grit.'—The 'Lower Yoredale Grit' forms a lenticular mass of grits and sandstones, with shales and ironstone interbedded. By the officers of the Geological Survey it was regarded as lying at the base of the black Bolland Shales, but by Messrs. Hind and Howe is included in the latter. By these authors it is also termed the Lower Yoredale or Pendleside Grit. The beds are local, although acquiring a thickness of 750 feet at Weets, immediately west of the Great Barnoldswick Fault. The topmost bed is well shown in Little Mearley Hall Clough, where it forms a well-marked conglomerate.
The Bolland Shales on the northwest side of Pendle Hill are about 700 feet thick, and consist mainly of black shales. They are usually calcareous, very fissile, and full of flattened fossils in a poor state of preservation. In the thin ironstones which accompany the shales the fossils are better preserved and uncrushed. The shales are very bituminous and not unfrequently smell strongly of rock oil. This bituminous character has in the past often led astray coal seekers, who have been convinced that the beds belonged to the coal measures, the shales of which they so much resemble. Not merely is there a superficial resemblance, but many of the fossils of the Bolland Shales are identical with those of the Lower Coal Measures; amongst these may be noted Posidoniella lævis, Orthoceras, Goniatites, and fragmentary fish remains.
- ↑ 'The Geological Succession and Palæontology of the Beds between the Millstone Grit and the Limestone-Massif at Pendle Hill.' Quart. Journ. Geol. Soc. 1901, lvii. 348.
- ↑ Prof. E. Hull, 'The Geology of the Burnley Coalfield,' Mem. Geol. Survey, p. 17.
- ↑ Op. cit., p. 349.
- ↑ Hind and Howe, op. cit., p. 352.
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