64 SECTIONAL ADDRESSES 
Increase of the thickness zone by zone towards the centre in the southern 
Pennine area may be accepted as evidence of progressive geosynclinal 
development and its differential deepening. ‘There is compounded with 
this regional settlement increase of thickness in troughs and thinnings 
towards the crests of local folds. Probably the best-known coalfield 
example of such local variability is the Potteries syncline and the adjoining 
Western or Rearers anticline of North Staffordshire. ‘There from crest 
to trough the total thickness of Productive Measures underneath the Red 
Beds varies within two miles from 2,500 to 3,500 ft. Individual coal 
seams continue across the whole coalfield ; marine bands and shales with 
non-marine lamellibranchs are similarly persistent, and the variable 
component in the measures is the coarse land waste which accumulated 
in thicker and more numerous sandstone lenses towards the centre of the 
trough. There is no evidence that the crest of this fold was so uplifted 
that some beds might have been denuded ; and it follows that the develop- 
ment of the anticline was by differential sinking of the lateral troughs. 
As with the regional tilting towards the centre of the geosyncline, so in 
this local folding the rate of change of thickness was slow until the more 
important coals had grown (Ovalis and Modiolaris zones), but becomes 
increasingly differential through and towards the later part of the Similis- 
Pulchra zone. 
The Horseshoe anticline in the North Wales Coalfield has a slightly 
different history. It too was sinking more slowly than neighbouring 
areas when measures containing the Lower Coal series were deposited 
across it. Thereafter, though it continued to receive a share of sediment, 
it maintained a line of shoals which acted as a barrier and diverted the 
flow of sediment, so that there is striking dissimilarity between the 
Bulkeley Fireclay series formed in the troughs of Flint, and the Upper 
Coal and Cefn Rock series deposited contemporaneously in Denbighshire. 
At its northern end along the Dee estuary, before the deposition of red 
Upper Coal Measures over it, some hundreds of feet of measures were 
denuded from its broken and upraised crest. Series of wash-outs in 
coal seams under sandstones in the Flintshire syncline are interpreted 
as erosion features produced by stream-courses directed longitudinally by 
the folding of the trough. 
In Coalbrookdale, Productive Measures, including some hundreds of 
feet of measures newer than the Pennystone, were sharply folded and 
faulted by posthumous disturbances of pre-Carboniferous post-Silurian 
folds. 'These movements were mainly completed and the anticlinal crests 
denuded before the overstep of Upper Coal Measures, which is the 
unconformity of the Symon ‘ Fault.’ 
Statistical studies of colliery records within the open folds of Yorkshire 
are bringing evidence that above the Barnsley Coal, and especially towards 
the top of the Similis-Pulchra and through the lower parts of the Phillipsii 
zones, the total thickness and the proportion of sandstone in the sedi- 
mentary column increases progressively to a maximum over the deepest 
parts of the Frickley and Maltby troughs.® ‘There is no suggestion of 
® “Sections of Strata of the Yorkshire Coalfield,’ Midland Inst. of Min. Eng. 
(1927). 
