FOREST OF WYRE AND TITTERSTONE CLEE HILL COAL FIELDS. 1067 



among which the abundance of sea-urchin plates is remarkable. The level of this 

 marine band, of which 8 feet is exposed, is between 150 and 300 feet above the base 

 of the group. 



Thickness. — The Cornbrook Sandstone nowhere displays its complete thickness, 

 for, though in places its true base is present, its summit is not preserved. Owing to 

 paucity of exposures, the thickness at many places along the outcrop is uncertain. 

 In the Cornbrook dingle, however, it is evidently little, if anything, less than 1000 

 feet ; but on the other side of Clee Hill it has been reduced to a few feet, and, close 

 by, the group disappears completely beneath the Coal Measures, which, as we shall 

 presently see, rest on it with marked unconformity. 



Relations to Older Formations. — The Cornbrook Sandstone is known to rest at 

 different places on the Lower Old Eed Sandstone and on the Carboniferous Limestone 

 Series. Its junction with the Lower Old Red Sandstone is sharp. That it is an 

 unconformity is undoubted, though at the single exposure known a discordance in 

 dip has not been demonstrated. The junction with the Carboniferous Limestone 

 Series, which is of greater interest, is, on the other hand, undoubtedly a conformity. 

 In the first place, wherever the junction is exposed the Cornbrook Sandstone 

 rests on one particular bed of the Carboniferous Limestone Series, and this bed is 

 a soft, red clay which would at once show signs of erosion if the sandstone above 

 were unconformable. The clay, though thin, persists all along the outcrop of 

 the junction, a matter of several miles, being present, in fact, even ivhere the whole 

 Carboniferous Limestone Series has thinned away laterally to a thickness of a few 

 yards. Again, the junction shows some of the features of a passage, though there 

 is no interbedding ; the top of the clay below is sandy, and the base of the sandstone 

 above is even. 



The Cornbrook Sandstone overlain by the Coal Measures unconformably. — The 

 junction of the Cornbrook Sandstone with the Coal Measures is of still greater interest, 

 for it is a marked unconformity. It is clearly exposed beside Benson's Brook on the 

 north side of the hill, where the upper formation, its lowest beds consisting of coarse 

 conglomerate, rests with gentle dip on the edges of the sharply dipping Cornbrook 

 Sandstones, the discordance amounting to 30° or more. 



Age of the Cornbrook Sandstone. — As we have seen, Dr Vaughan has inferred 

 that the Cornbrook Sandstone is of Upper Avonian age. This conclusion has been 

 confirmed by more recent work, which has shown (l) that the Sandstone is truly 

 conformable with the Carboniferous Limestone Series below, the highest recognisable 

 horizon of which belongs to the top of the Lower Avonian ; (2) that the flora of the 

 Sandstone so far as known is Lower Carboniferous ; and (3) that the Sandstone is 

 separated from the Coal Measures by a marked unconformity. It may be added that 

 the fossils so far obtained from the marine shale and limestone in the Cornbrook 

 Sandstone are of no zonal value. 



