102 T. Davidson & G. Maw — U. Silurian Rochs of Shropshire. 



enabling the eye at a glance to follow all the main subdivisions. 

 Standing on Benthall Edge or Wenlock Edge, the most prominent 

 points in the escarpment, three parallel valleys and two well- 

 marked intermediate ridges can be made out at almost every part of 

 the long line of exposure extending from Ironbridge on the north- 

 east to Ludlow on the south-west, the three valleys corresponding 

 with the soft shales and the two ridges with the limestones. 



The broad sweeping valley of Ape Dale below the observer to 

 the north-west represents the Wenlock Shale, backed up on its 

 north-western side by the harder beds of Llandovery Limestone and 

 conglomerate forming the base of the Upper Silurian series. 



The Llandovery beds on the lines of section may be roughly 

 estimated at a thickness of 160 to 170 feet, of which the con- 

 glomerate, closely resembling the Millstone Grit, forms the greater 

 bulk. The overlying Wenlock series attain a thickness of from 

 2500 to 2800 feet : their principal mass consists of soft shales 

 capped by the Wenlock Limestone, which has determined the 

 beautiful escarpment of Wenlock Edge, overhanging the gentle 

 sweep of Ape Dale, and on the south side forms a regular dip 

 slope into the Lower Ludlow valleys of Much Wenlock and Hope 

 Dale. 



No clear line of boundary exists between the limestone and shale, 

 for the one imperceptibly graduates and dies out into the other. 



From careful measurements made on Benthall Edge we have 

 ascertained that the compact limestone is from 80 to 90 feet in 

 thickness, and it thickens somewhat in the direction of Wenlock to 

 the south-west. Below the compact limestone rock it becomes inter- 

 stratified with thin layers of shale. Still lower down it assumes 

 a concretionary structure, and gradually dies out into soft shale, 

 through increasingly distant nodular courses, at about 400 or 500 

 feet below the crest of the limestone ridges. 



On Benthall Edge the Wenlock Limestone dips from 15° to 20° 

 S.S.W. ; to the westward the dip decreases to from 10° to 15°, and at 

 the eastern extremity of the escarpment at Lincoln Hill, near Coal- 

 brook Dale, the inclination increases to from 45° to 50°. The upturn- 

 ing may have been continuously gradual or interrupted. It commenced 

 'before the Carboniferous period, for the Coal-measures rest upon it 

 unconformably, and it continued subsequently, indicated by the fact 

 that the inclination of the margin of the Carboniferous beds is 

 related to the greater or less inclination of the subjacent Wenlock 

 Limestone. 



The following proposed subdivision of the great mass of Wenlock 

 Shale, which at the north-eastern end of the Shropshire escarpment, 

 attains a thickness of from 2000 to 2200 feet, has been suggested by 

 the alternation of zones of highly fossiliferous and comparatively 

 barren strata. 



As stated above, there is an insensible gradation between the 

 Wenlock Limestone proper and the Wenlock Shale, the shales 

 under the limestone containing scattered concretionary courses of 

 nodular limestone, and it will be convenient to term this intermediate 



