69 



Silurian Strata in the Mendips. 



By Dr. W. T. Ord, Chairman of the Geological Section. 



{Presented to the Members atteitding the Geological Excursion 

 to the MendipSj on July 14th, ^909)' 



The discovery, by Prof. Reynolds of the Bristol University, of 

 Silurian strata underlying the old red sandstone in the Mendip Hills, 

 is one of the most important and interesting contributions to the 

 geology of England that has been made for some years. The way 

 in which this discovery was made, and the facts which led Prof. 

 Reynolds to identify Silurian beds, affords a striking example of the 

 romance of a science, which many people imagine to be dulness 

 itself and very much the reverse of romantic. 



To make clear how this discovery was made, a brief account of 

 the geology of the Mendips must be given ; this will be much 

 simplified by a study of the section given in Plate VII. The Mendip 

 Hills consist of four great folds of Carboniferous Limestone, arranged 

 en echelon from north-west to south-east. Each fold, called in 

 geological terms a pericline, contains a core of Old Red Sandstone, 

 and probably of Silurian beds beneath, although this has only been 

 demonstrated so far, in the case of the south-eastern pericline, which 

 is known as Beacon Hill, and is the one with which this paper is 

 concerned. This hill has the further peculiarity of exhibiting a 

 series of igneous rocks, which is shown in the section on the Plate. 

 After the close of the carboniferous period in geological times, the 

 sea-bed was raised up, the strata folded and bent up, and finally a 

 great part of the carboniferous rocks and most of the coal measures 

 were denuded off the land. On the planed edges of this strata, after 

 subsequent immersion, horizontal beds of more recent strata, rhaetic, 

 lias, and inferior oolite were laid down. These now appear on the 

 surface of the Mendip area, except near the higher grounds, where 

 the Carboniferous strata, and in some parts the Old Red Sandstone 

 crop up. A glance at the plate will make this clear. 



We are now concerned with the igneous rocks of Beacon Hill, 

 in connection with which the Silurian strata were discovered. 

 These igneous rocks are shown in the Geological Survey Map 

 (sheet 19) as rising along the ridge of old red sandstone to the north 

 of Shepton Mallet. They were formerly supposed to be lavas 

 which had broken through the adjacent strata from beneath and 

 spread themselves over them, having subsequently been denuded 

 down to their present extent. But Prof. Reynolds has shown that 

 they are probably the remains of volcanic eruptions which occurred 

 at an earlier period, some certainly during Silurian times. These 

 igneous rocks have for years been extensively quarried for road 

 metal, and it was in the Sunny Hill quarry that proof of their age 

 was first found. Amidst the great masses of trap or lava which are 



