26 Notices of Memoirs — Professor T. Franklin Sibly — 



main uplift of the May Hill anticline lying immediately east of the 

 Forest of Dean. ITorth-and-soiith folding predominated, but was 

 accompanied by some east-and-west folding. The result was a basin, 

 markedly unsymmetrical in structure, on the site of the present 

 coal-field. Along what is now the eastern edge of the coal-field the 

 Lower Carbonif«rous strata were involved in the western limb of the 

 May Hill anticline, and acquired a steep dip westwards, the larger 

 part of the very steep dip that they possess to-day. Westwards 

 across the site of the present coal-field, away from the May Hill 

 axis, the intensity of folding diminished very rapidly, and on the 

 western side of the basin the inward dip of the strata was very 

 slight. Consequently, the beds of the Coal Measures are nearly, but 

 not exactly, accordant with the underlying strata on the western side 

 of the present coal-basin, but markedly discordant with them on the 

 eastern side. Hut, slight though the discordance may be on the 

 western side, the behaviour of the outcrops supplies convincing 

 evidence of unconformity all around the coal-tield. The base of the 

 Coal Measures pays no regard to the strike of the Lower Carboniferous 

 beds, but everywhere passes to and fro, slowly or rapidly, across 

 their outcrops. 



Two interesting and significant features are (1) the development 

 of conglomerates at the base of the Coal Measures, and (2) the con- 

 cealment of the Treuchard Coal and the measures beneath it by the 

 overlap of the overlying measures, on the south-eastern border of the 

 coal-field. These may be described in connexion with the uncon- 

 formable overstep of the Coal Measures. 



The lowest beds of the Coal Measures, those underlying the 

 Trenchard Coal (Upper Trenchard Coal in some parts of the coal- 

 field), were termed Trenchard Measures by the late H. D. Hoskold.^ 

 The Trenchard Measures, although variable in character, usually 

 consist largely of yellow grits, in part fine-grained, compact, and 

 well-bedded, in part coarse-grained, friable, and imperfectly 

 stratified. The intercalated clays are sometimes mottled in purple 

 and yellow. A characteristic feature of these grits, particularly in 

 the coarse-grained and conglomeratic varieties, is the abundance of 

 an indurated, white or yellow clay cementing the grains. On the 

 northern and north-eastern borders of the coal-field these grits of the 

 Trenchard Measures often become very coarse-grained and pebbly at 

 their base, and bands packed with quartz pebbles or quartzite pebbles 

 constitute well-defined basal Coal Measure conglomerates. 



On the northern edge of the coal-field, between Drybrook and the 

 Lydbrook Valley, the base of the Coal Measures transgresses the 

 older strata rather sharply, the Drybrook Sandstone and the upper 

 beds of the Carboniferous Limestone are concealed, and the grits of 

 the Trenchard Measures rest directly upon Carboniferous Limestone. 

 A quarry 1,100 yards east of Ruardean Church shows masses of 

 coarse, pebbly grit resting upon, and in places "piped" down into, 

 the dolomites of the Carboniferous limestone. The former extension 

 of Coal Measures northwards and westwards across the denuded 



^ "Geological Notice upon the Forest of Dean" : Proc. Cotteswold Nat. 

 Field Club, vol. x, pp. 123-77, 1892. 



