Geology of the Forest of Bean. 25 



The upper portion of the Carboniferous Limestone of areas to the 

 south is represented in the so-called Millstone Grrit of the Forest of 

 Dean. This Millstone Grit, which succeeds the Carboniferous 

 Limestone quite conformably, is mainlj-, if not entirely, a formation 

 of Lower Carboniferous age. The name Dry brook Sandstone was 

 applied to it in 1912. 



Thick bands of limestone and dolomite appear in the Drybrook 

 Sandstone on the south-western margin of the coal-field. Near 

 Milkwall oolitic limestones in the lower part of the formation have 

 yielded Seminula ficoides, Cyrtina carbonaria, and other fossils of the 

 main Semmiila zone (S2). Cyrtina carbonaria has also been observed 

 in corresponding beds of dolomite in the Parkhill adit (Frj^er's 

 Level). The lower portion of the Drybrook Sandstone may, there- 

 fore, be correlated definitely with the main Seminula zone of the 

 Carboniferous Limestone. Unquestionably, the Drybrook Sandstone 

 passes laterally into limestones as we proceed from the north-eastern 

 margin of the Forest of Dean southwards to Chepstow and Bristol. 



Concurrently with the development of limestones the arenaceous 

 beds, which compose the bulk of the Drybrook Sandstone even on 

 the south-western margin of the coal-field, become finer in grain 

 Avhen followed south-westwards. For example, seams of quartz- 

 conglomerate are conspicuous in the Drybrook Si^ndstone of the 

 Mitcheldean disti'ict, but these have dwindled to insignificance in 

 the neighbourhood of Bream. Bands of shale and fine-grained 

 sandstone, containing shreds of coal, are found in the upper part of 

 the Drybrook Sandstone in the Parkhill adit. 



Owing to overstep by the unconformable Coal Measures the 

 Drybrook Sandstone is wholly concealed both on the south-east 

 between Lydney Park and Staple Edge Wood, where the Carboni- 

 ferous Limestone also is concealed, and on the north between 

 Drybrook and Lydbrook Valley. From the same cause the apparent 

 thickness of the Drybrook Sandstone varies greatly, and in no 

 regular manner, along its outcrop. The thickness is at least 

 650 feet in the Soudley Valley between the Shakemantle Pit and 

 Staple Edge Halt, where the upper beds are well exposed on the 

 railway. 



The Coal Measures of the Forest of Dean lie unconformably, and 

 sometimes with great discordance of dip, upon an eroded floor formed 

 by the Drybrook Sandstone, the Carboniferous Limestone, and on the 

 south-eastern margin of the coal-field, the Old Ked Sandstone. 



This important unconformity is due to an intra-Carboniferous 

 episode of crust-movement, folding, and denudation which followed 

 the deposition of the Drybrook Sandstone, but preceded the formation 

 of tlie existing Coal Measures of the Forest. The latter were 

 deposited on the denuded edges of the older strata. An altogether 

 later movement involved the Coal Measures, gave them their present 

 basin-like arrangement, and served also to accentuate the folding 

 previously imposed upon the older rocks. 



The intra-Carboniferous disturbance responsible for the uncon- 

 formity necessarily involved the Silurian and the Old Red Sandstone, 

 together with the Lower Carboniferous strata. It produced the 



