Geology of the Forest of Dean. 27 



edges of the underlying strata is proved by outliers of Trenchard 

 Measures. In the large outlier of Howie Hill, represented as 

 Millstone Grit on the Greological Survey map (sheets 43 S.W. and 

 43 S.E., Old Series), Trenchard Measures rest directly upon the 

 Lower Limestone Sliales. A smaller outlier on Courtfield Hill, 

 Welsh Bicknor, not shown on the Survey map, rests upon the Lower 

 Limestone Shales and the base of the Main Limestone. 



The railway-cutting immediately north of Drybrook Halt gives 

 a fine section of Trenchard Measures resting upon massive sandstones 

 which lie in the lower part of the Drybrook Sandstone. The basal 

 beds of the Coal Measures is here a remarkable pebble-bed with 

 large, well-rounded pebbles of grey quartzite. This pebble-bed has 

 been traced some distance north of Drybrook, and has been recognized 

 in the Howie Hill outlier. 



On the eastern edge of the coal-field from Wigpool Common as far 

 south as the Soudley Valley, the Trenchard Measures rest upon 

 Drybrook Sandstone. In the Soudley Yalley, the railway-cutting 

 south of Staple Edge Halt exposes the unconformable contact of the 

 two formations. Conglomerates forming the base of the Trenchard 

 Measures, and containing fragments of a fine-grained, white sand- 

 stone which can be matched in the Drybrook Sandstone of the same 

 cutting, rest upon the Drybrook Sandstone with discordance of dip. 

 The average dip of the Drybrook Sandstone in the cutting is 

 50° W.N.W. The conglomerates dip slightly north of west at 

 about 25°. 



South of the Soudley Valley, overstep carries the base of the 

 Coal Measures southwards, and then eastwards, across fully 650 feet 

 of Drybrook Sandstone and tlie whole of the Carboniferous Limestone, 

 in the distance of bai'ely 2 miles to the southern side of the 

 Blackpool Valley. The Lower Carboniferous strata maintain a steep 

 north-westerly dip, rising to 65° in places, as their strike swings 

 gently from S.S.W. to S.W. The Coal Measures maintain a moderate 

 dip a little north of west. The Drybrook Sandstone and the upper 

 beds of the Carboniferous Limestone are transgressed gradually in 

 Staple Edge Wood. The bulk of the Carboniferous Limestone is 

 transgressed very sharply in the Blackpool Valley. On the south 

 side of that valley the base of the Coal Measures continues its rapid 

 overstep eastwards until, just north of Danby Lodge, it crosses the 

 quartz -conglomerates which lie some 400 feet down in the Old Red 

 Sandstone. 



In consequence of this sharp overstep the Carboniferous Limestone 

 and the Upper Series of the Old Red Sandstone remain wholly 

 concealed from Danby Lodge to the western side of the Cannop 

 Valley, above Lydney. In and near Stonebury Wood, north of 

 Lj'dney Park, the quartz-conglomerates of the Old Red and the beds 

 of the Carboniferous Limestone, diy)ping very steeply westward, 

 reappear from underneath the cover of unconformable Coal Measures. 

 The sharp swing of the Coal Measure base here carries it back 

 rapidlv from the Old Red Sandstone on to the Drvbrook Sandstone 

 in Old Park AVood. 



Tlie unconformable overstep of the Coal Measures is attended, on 



