History of Madeley / including Ironbridge, Coalbrookdale, and Coalport

by John Randall

The descent from the Crawstone crust to the Silurian shale of the Dale cannot be traced. As passed through at the Limestone pit at Lincoln Hill, it was as follows:—

		Ft.	In.
1.	Crawstone Measure Crust	1	8
2.	Rock	10	6
3.	Coal Smut	0	9
4.	Clunch with balls of Sandstone	12	0
5.	Lancashire Ladies’ Coal	0	6
6.	Strong Clunch with Sandstone balls	19	0
7.	Sandstone Rock	10	6
8.	Chalkstone	12	0
9.	Limestone (Silurian)	28	6
Total		95	5

This then may be considered a fair representation of the remainder of the measures which occur below those seen on the surface at the Black Rock Quarry; but the passage from the carboniferous to the Silurian formations is no where conformable, and no mention is made of the Millstone grit, a portion of which certainly intervenes, and which is to be seen in small patches near, but which might possibly be represented by the three or four last measures in the section of the Lincoln Hill Limestone Shaft.

Excellent opportunities occur in this immediate neighbourhood of studying the junction of the Silurian and Carboniferous formations, and of the evidences afforded of the denudation of the one prior to the formation of the other. To the general reader these words may convey little meaning, but the scientific student who studies the evidences here made clear cannot fail to comprehend the fact that he has before him not only an old sea-bed, rich in relics of the fauna which inhabited its waters, but a sea-bed which had become a cliff, and had in turn been gradually cut down and wasted during successive ages prior to that at which a carboniferous flora had begun to flourish. Two series of rocks are here in juxtaposition, yet so widely separated by time, as to indicate a gap in the consecutive history of the earth as great as if we were to blot out the intermediate history of this country from the close of the Heptarchy to the reign of George III.; only that the period of time in the latter case would bear no manner of comparison with the former. If we suppose the Wenlock limestone to have been once covered at these points by the Ludlow limestone, and this again by the old red sandstone—as is the case to the south, to say nothing of the carboniferous limestone and millstone grit, we are forced to the conclusion that thousands of vertical feet, and hundreds of cubic miles of solid ground were first piled up and then cut down and carried away by the sea. Creation itself in the interval of their formation passed through many phases, during which new species came slowly into being and disappeared, and were again replaced by others. To fill up the gap that succeeds this great silurian flooring of the coal-measures, to study the intermediate links of the missing strata we must go to the millstone grit in its undenuded or partially denuded state, as it occurs beneath the coal-measures of Little Wenlock, or at the bend of the road, called “The Turn,” in going from Coalbrookdale to Wellington.

The first thing striking the attention is a buff coloured shale, weathered on the surface to clay, at the base of the bold bluff cliff of gritty sandstone so conspicuous on the brow of the hill. Whilst examining this member of the Silurian series a man from a neighbouring cottage remarked, “That is fuller’s earth; persons fetch it when they are galled, and it is good for the eyes; large quantities are fetched away and sent to Manchester.”

The fossils it contains show that it belongs to the lowest member of the Ludlow group, and that the whole of the Aymestry and Upper Ludlow have been stripped off and washed away before either the millstone grit or the coal-measures were formed.

Among the fossils yielded by this shale, in addition to bivalves and corals, are those interesting forms of crustaceans called pen fossils, from their resemblance to a quill pen. The species we found was Graptolithus priodon, described in the early works of Murchison as Graptolithus Ludensis. The trilobites, from the fineness of the material, are so sharply and beautifully preserved that the visual organs of the little creatures are clearly discernible, even to the optical tubes, elongated cones, or crystaline lens such as are to be seen so marvellously distinct on the eyes of the dragon-fly of the present day. The beautiful markings too on the shield of these wondrous little creatures which flourished in these seas, in such numbers that they may be got out in groups—forms which died out and perished before the close of the carboniferous formation represented above it—are so delicate and fine as to equal if not to defy imitation in ordinary materials in use at the Dale Works; and it is we fancy at least worth the experiment whether with this shale reduced to powder it might not be made to produce delicate impressions after the example here set by nature. We also found here some beautiful Lingula, a Patella, an Orbicula, a Leptæna, a Lituites, a Fienestella, and other fossils. To inhabitants of the Dale, here is a field of research open which they may make their own, close to their own doors.

This fine earth is known by various names where it occurs in Shropshire and the adjoining county of Herefordshire. In the latter county it is said to be used by country people for cleansing purposes, in which case it is called “Walker’s earth or soap.”

If the reader will follow this soft soapy shale, as we did, higher up into the coppice, he will find large masses of rock which have been toppled over through the shale giving way. A slip on the side of the narrow path discloses a bed of it, and immediately above it, consequent upon a former slip, we come upon a sandstone rock from twelve to fourteen feet thick, with quartz pebbles, representing the millstone-grit. Then a bed of black shale occurs, about six inches thick, which is chiefly made up of coal-plants, some of which are converted into charcoal. These plants do not appear to have grown on the spot, but to have been drifted into their present position. They were evidently in a soft and yielding state, some of them being pressed quite flat. [318] One good sized slab opened with a cast of a Lepidodendron, and we met with another cast, clearly of the same tree, a short distance west of it. Another, a Sigillaria, was much more distinct, the leaf scars being quite sharp, and the fibres of the inner bark very clear. This interesting band of coal shale is succeeded by another of yellowish clay, of about equal thickness; and these are followed by a second and a third band of black shale, with alternate ones of yellowish clay. Above these are thick sandstone rocks, some white, and some coloured red by iron, which here and there occurs in the form of hematite.