Page:VCH Herefordshire 1.djvu/74

A HISTORY OF HEREFORDSHIRE and from which a magnificent view is obtainable over the undulating lowlands. To the botanist the district where the Carboniferous Limestone occurs is of interest because in it grow many plants not found in the Old Red low- lands, and the Great Doward is especially mentioned by Symonds as the habitat of several rare species. Although the Carboniferous rocks occupy so small a part of the county at the present time, there seems to be little doubt that at one time they extended over by far the greater portion. Mr. A. J. Jukes-Browne, in his valuable and suggestive writings on historical geology, pictures the sea in which the Carboniferous Limestone was formed as analogous to the West Indian and Mexican seas of the present day. He pictures an island of irregular outline as occupying at that time a considerable portion of the Midlands ; but his chart requires amend- ment. Certainly the Clee Hill area was connected with that of the Forest of Dean and Bristol, and such phenomena as obtained on the borders of Gloucestershire at Newent of the Coal Measures resting directly on the Old Red Sandstone may just as well be interpreted as pointing to the great amount of denudation that took place between the time of forma- tion of the older and Upper Coal Measures of the Midlands, as to the presence of a land-surface while the missing Carboniferous rocks were being deposited elsewhere. While limestone was still being deposited in the Bristol district changes were impending in the Clee Hill area, for there there are indications of gritty rocks at a comparatively early date in the Avonian Epoch. From the fact that the Millstone Grit facies sets in earlier in the Clee Hill area than in the Forest of Dean, and earlier in the later district than in the neighbourhood of Bristol, it is inferred that a change, resulting in the shallowing of the sea, took place, which initiated conditions suitable for that luxuriant growth which is now compressed into so many seams of coal. It was after a considerable thickness of Coal Measures had been formed that the crust-pressures referred to above acted, and it was then that the Malvern Hills were elevated, according to Professor Groom, in sections proceeding from north to south with the greatest uplift in the south ; but he also thinks that it was not until post-Liassic times that they attained anything like their present definition. The crust-pressures so affected the geographical conditions that a lacustrine area was formed over the Midlands, in which the Upper Coal Measures of those parts were laid down. How far this lacustrine area extended into Herefordshire cannot now be ascertained, because apart from the enormous denudation that has taken place in comparatively recent geological times, there was considerable Pre-Permian denudation. This is known to have been the case from the relations of the Permian rocks to the subjacent beds in certain of the Worcestershire sections. According to Professor Groom, this Pre-Permian denudation removed the Upper Coal Measures ' nearly or quite to the level of the old plain on which they were deposited, and in most cases denudation proceeded still further, so that the Haffield Breccia rests directly upon the older rocks.' 26