74 SECTIONAL ADDRESSES. 



of the overlap, and thus lead to a closer estimate of the probable maximum 

 height to which the base of the formation ascends. From the central 

 Mendips the Lias, resting in general upon Rhaetic, declines somewhat 

 rapidly southward to a level of about 500 feet around Wells and Shepton 

 Mallet, and must reach the level of the sea three to four miles south of 

 Wells. 



At Glastonbury Tor both Middle and Upper Lias overlie the Lower 

 Lias, and the same succession is found to the east in the Pennard ridge. 

 According to the section given by Woodward the base of the Lias at 

 Glastonbury must be about 200 feet below Ordnance Datum, but south- 

 wards in the Polden Hills it rises again to between 200 and 300 feet 

 above O.D. 



Immediately north of the Mendips the basal plane also descends to 

 considerably lower levels, and in the outliers of Lias and Rhaetic west of 

 Chew Stoke it stands at approximately 400 feet above O.D. Still farther 

 north, however, around the North Hill Carboniferous inlier, the base of 

 the Lias rises again to about 600 feet above O.D., thence declining north- 

 ward to less than 100 feet along the valley of the River Avon between 

 Bristol and Bath. North of that valley there is a gradual rise of the 

 base to 200 feet O.D. and in places to 350 feet. On a line of section drawn, 

 therefore, from the northern suburbs of Bristol through North Hill, to 

 Glastonbury and beyond, we have two well-marked regions of elevation, 

 viz. one in the North Hill region and the other in the Central Mendips ; 

 and three synclinal depressions, viz. at Glastonbury, between North Hill 

 and the Mendips, and in the Avon valley. The difference of elevation 

 between the highest and the lowest points amounts to over 1,000 feet, and 

 as the junction of the Rhaetic and Lias must have been as nearly as possible 

 level at the time of the deposition of these rocks, the difference is a measure 

 of the movements which the strata have suffered since they were deposited. 



By using all the data available on the geological maps of the Bristol- 

 Mendips region, I have drawn strike lines showing the approximate level 

 of the basal plane of the Lias (see Plate II). On this map certain 

 structures are immediately apparent ; a well-defined anticlinal fold 

 coincides approximately with the crest line of the Mendip range ; north 

 of it another anticline in North Hill which apparently continues eastward 

 under Dundry Hill. Between these two areas of elevation there is a 

 depressed region in which the structure is less clearly defined, though at 

 all points the base of the Lias is at a lower level than to the north or 

 south. Within it there appear to be two shallow synclines between 

 which there is a subsidiary anticline near Farmborough. The small 

 synclinal outcrop of Lias near Banwell at the western end of the Mendips 

 apparently lies in the westward continuation of this depressed area. 

 South of the Mendips the deep Glastonbury syncline with its east-west 

 trend is readily detected. 



So far as the evidence goes, the anticlinal areas appear to pitch east 

 and west so that they are elongated domes. Unfortunately, the absence 

 of the Lias along the west end of the Mendip range renders the course of 

 the strike-lines uncertain, but the convergence westward of the Trias 

 outcrop and the approach in that direction of the 100 foot strike lines, one 

 on the south and the other on the north side of the range, makes it 



