484 



WENLOCK LIMESTONE.— THE WREN'S NEST. 



Ground plan and transverse section of the Wren's Nest, the latter double the scale of the former. 



e dc I a N 



M 



102. 



Ground Plan 



Section on 



Ww///iW////£m§^ 



e f 



HI 

 deb 



These upper and lower limestones (-6 and d) constitute, however, 

 only part of this formation, the remainder, as at Wenlock, heing 

 made up of innumerable small concretions of impure argillaceous 

 limestone, occurring at intervals in a matrix of grey shale like the 

 flints in chalk. These concretions are, however, in parts so abun- 

 dant as almost to constitute one entire calcareous mass, and in 

 districts where lime is scarce they would doubtless be separated 

 from the shale and burnt. Here, however, they are considered as 

 refuse. They are called " bavin,'" the shale associated with them 

 being termed "rotch." Above the upper limestone (b) these impure calcareous beds (a) are about 

 100 feet thick ; between the limestones they are about 90 (c), and beneath the lower limestone 

 about 60 (e) 3 so that the entire mass, including the impure beds, exceeds 300 feet. This is exclusive 

 of the lower part of the formation or Wenlock shale (/), which is more than double this thickness 5 

 and therefore the whole formation at Dudley may be estimated at 1000 feet or more. 



The limestone is lithologically undistinguishable from that of Wenlock, like which it occasionally 

 runs into large concretions, less argillaceous and more crystalline than the mass. These concretions, 

 the " ballstone " of the Shropshire workmen, (p. 210.) are here called " crogs," and they present just 

 the same appearance of interfering with the stratification which has been described near Wenlock. 



Organic remains occur at intervals throughout all these beds, but the most beautifully preserved 

 specimens and the greatest variety, appeared to me to lie on the surface of certain thin flaglike beds 

 of the upper limestone. 



The elliptical shaped mass of the Wren's Nest is, therefore, composed of an exterior 

 mantle of pure and impure limestone, and a nucleus of inferior shale, in which, as at 

 Wenlock, all calcareous matter disappears. The former rises around the latter at high 

 angles of inclination, i. e. at about 60° on the eastern face and 45° on the western. The 

 purer limestones have been long extracted from the hill itself, as represented by the 

 hollow spaces in the above section ; and if the impure limestone or " bavin " had been 

 worth removing, the Wren's Nest itself would long ago have been demolished, leaving 

 only a central mound of shale to mark its former existence. The ridges of ''bavin" 



