Br. R. L. Sherlock — Datum-lines in English Keuper. 121 



have already been published.^ The two latter mines and quarries 

 belong to The Vale of Belvoir and Newark Plaster Company, the 

 others to Messrs. Cofferata & Co., and my thanks are due to these 

 two firms for the kindness with which they have allowed me to visit 

 the sections, and for information. The distance from Beacon Hill, 

 Newark, to Hawton is about 2 miles ; Hawton and Bowbridge are 

 only separated by some 350 yards, Bowbridge lies slightly to the 

 east of Hawton, and in consequence of the easterly dip higher beds 

 are visible at Bowbridge than at Hawton. From Bowbridge to 

 Orston is about 7 miles. The Orston mine was at the time (1915) 

 disused and the details of the section were given by the proprietor. 

 Except at Oi'ston the seams of gypsum are visible throughout the 

 extensive sections (the Hawton quarry is about 700 yards long), and 

 occur in the same order and at approximately the same distance 

 apart, and there is no doubt that they are definitely bedded deposits. 

 Additional evidence of this is furnished by the fact that some of the 

 accompanying beds can be correlated in different sections. Thus, the 

 "Biders" (see Sections, p. 122), a nodular band of gypsum resembling 

 a line of flints in chalk, occurs between the Top and Middle White 

 Rocks, at Newark, at Hawton, and at Bowbridge ; while the 

 "Bastard", a 3 ft. band of mixed green marl and gypsum, occurs 

 7 feet above the Top White Rock at the same three localities. 



At Beacon Hill, Newark, some 66|- feet of strata intervene 

 between the Top White Rock and the base of the Rhaetic beds.^ At 

 Bowbridge there are 45 feet of strata exposed above the rock, and, in 

 addition, a certain amount crops out under alluvium between the 

 section and the Rhaetic escarpment. At the constant dip prevalent 

 over the district there is room for about 20 feet of strata in this gap, 

 giving approximately 65 feet of strata up to the Rhsetic base, or 

 practically the same as at Newark, 2 miles away. At Orston there 

 is recorded 39 feet of strata above the Top White Rock. Un- 

 fortunately the section cannot now be seen, and the exact thickness 

 of strata cropping out between it and the Rhsetic base, cut through 

 in the railway about 20 yards away, cannot be measured exactly. 

 It would appear that the total thickness of strata between the Top 

 White Rock and the Rhaetic is a few feet less at Orston than at 

 Newark. 



This evidence shows that between Newark and Orston the Rhaetic 

 is separated from the Top AVhite Rock by a belt of strata of 

 practically constant thickness. In the absence of complete sections 

 it cannot be said that the thickness of the intervening strata is 

 absolutely constant, but it can be said that throughout the distance 

 of 9J miles the variation is not more than a few feet and may be 

 ■quite absent, aud that over a distance of 2 miles there is no variation 

 "whatever. Exact correspondence in thickness of the marl beds in 

 any two sections is not to be expected, as we may see from the 

 sections figured, and any small differences in the thicknesses of the 

 intermediate strata are more likely to be due to variation (and cancel 

 themselves out) than to an unconformity, however slight, of the 



^ A. J. Jukes-Browne, Geology of the South-West part of Lincolnshire 

 <Mem. Geol. Surv.), 1885, p. 18. 



