WENLOCK SHALE. 



213 



superior strata of this shale are well exposed at the escarpment of Lincoln Hill, and they 

 occupy the whole of Coal Brook Dale, properly so called, where they are cut through 

 by the road which leads to Wellington, the coal-measures resting on their highly in- 

 clined strata. The same beds are equally well seen all along the escarpment of the 

 Wenlock Edge, where they dip beneath and pass up into the lower nodular beds of the 

 limestone. In some places the laminae of deposit are indicated by large spheroidal 

 forms. In the central portions the shale is very incoherent, and where it has been 

 exposed to the atmosphere and not preserved by cappings of the harder limestone, it 

 has been much denuded. This incoherent structure has invariably led to deep denuda- 

 tion, which in this tract is indicated by the longitudinal valley extending from the 

 Severn to the Onny, in a line parallel to the Wenlock Edge, and between that ridge 

 and the Caradoc Hills. Similar valleys are seen at the back of the escarpment of the 

 Ludlow promontory, and indeed wherever this sobformation is exposed. 



Towards the base of this shale are sometimes courses of very impure limestone, which 

 have a lenticular or rather brick-shaped form. Such beds occur in the slopes of the 

 hills above Buildwas. Some of the spheroidal, argillo-calcareous concretions present 

 when broken, an internal structure similar to the well-known " cone in cone " of the 

 lias shale, the cones being frequently made up of dark-coloured crystalline carbonate of 

 lime in an argillaceous paste 1 . Similar nodules, of the size and form of large loaves of 

 bread, are sometimes found on the surface of the hills as we approach the Wrekin, and 

 the same have been recently detected 2 near Cound, where they are included in the 

 shale, which covers the Caradoc sandstone. 



The shale is usually succeeded by sandy, calcareous bands, charged with peculiar 

 fossils ; but as in most cases they rise up in separate ledges reposing upon, and passing 

 into the Caradoc sandstone, I have preferred to include them in that formation. As a 

 general rule, therefore, we consider the upper Silurian rocks to have their natural base 

 line, where the soft Wenlock shale ceasing, harder strata, for the most part sandy, but 

 sometimes calcareous, begin to rise out from beneath them. 



It is perhaps more difficult to calculate the entire thickness of the Wenlock forma- 

 tion than that of the Ludlow rocks, since many of its lower strata are much denuded 

 and ill exposed. The limestone in the vicinity of Wenlock, including all the superior 

 and inferior nodular strata, cannot have a less depth than two hundred feet : it possibly 

 may in some parts amount to three hundred ; and the thickness of the lower shale, 



the Silurian System which they meet with below their coal measures. If this term had been strictly limited 

 either to the shale above or to that below the Wenlock limestone, it might have been retained in geological, 

 nomenclature ; but it cannot be used, because the carboniferous deposits of that tract rest on various members 

 of the Silurian System, and hence the " Die Earth " beneath one shaft refers to the upper or lower Ludlow rock, 

 beneath another to the Wenlock shale. 



1 In shale of this age in Montgomeryshire some of the nodules contain crystals of quartz mixed up with those 

 of carbonate of lime, small flakes of anthracite, &c, and various organic remains. (See chapter 24.) 



2 On a farm of the Rev. Dr. Butler, now Bishop of Lichfield. 



2 D 



