110 A. B. Triieman — The Lias of South Lincolnshire. 



General CoNSiDEEATiojr of the Upper Lias. (Figs. 4 and 5.) 

 The variation in the thickness of the Upper Lias in Lincolnshire 

 has resulted from two movements, viz. : 



(1) A series of uplifts along an axis in South Yorkshire at 



intervals during the deposition of Lias and later rocks. 



(2) The migration of the area of maximum deposition of the 



Upper Lias fi-ora north to south. 

 This latter movement was traced by Mr. S. S. Buckman,' who 

 showed that the zones which are represented by thick deposits in 

 Yorkshire, are only present as thin layers further south, while later 

 zones not well developed in Yorkshire are very thick in the south. 

 This migration of the area of maximum deposition may now be traced 

 across Lincolnshire ; thus the tenuicostaturn subcarinatum sub-zones 

 attain their maximum thickness in Yorkshire, and are fairly thick in 

 Lincolnshire, but rapidly decrease in thickness towards Northampton- 

 shire and the south. On the other hand, the fihulatum zone, which 

 is only thinly represented in Yorkshire, shows increasing thicknesses 

 at Grantham and Northampton. In this area it is interesting to 

 notice that the places of minimum deposition in any zone are 

 characterized by Oolitic beds. It is probable that during the 

 deposition of the Upper Lias a shallow down fold passed gradually 

 from Yorkshire southwards, its position determining the area of 

 maximum deposit at any time. 



' ■••■■'.• '■•.'■.■*■'•." I .. I .1. . . I.I . i .L I I.I ■ 1 . .1 11 W»'^WI» I. . . 1.1. J ■ I . . J I I I -■■■■ r«. 



foit 



Villi sub'-une 



^ub -20(16 



Subtarinal'ttn 



Fig. 5. — Diagram showing the relationship of the Northampton Sands and 

 Upper Lias. (Not to scale.) 



Accompanying this movement was one which probably commenced 

 earlier, that is, an uplift or rather a series of uplifts along an axis 

 in South Yorkshire. As a result of this the thickness of the Lias as 

 a whole decreases as it is traced southwards across Yorkshire or 

 northwards across Lincolnshire. The Upper Lias also decreases in 

 thickness in the same way ; in Northamptonshire it is about two 

 hundred feet thick, at Grantham 115 feet, and at Lincoln only one 

 hundred feet, diminishing still more rapidly farther north until at 

 Appleby 2 it is usually little more than fifty feet thick. The 

 thinning, however, is probably not so regular as appears from these 

 figures, for at Caythorpe, between Grantham and Lincoln, a boring 

 showed the Upper Lias to be nearly two hundred feet thick,* 



^ S. S. Buckman, " Certain Jurassic (Lias-Oolite) Strata of South Dorset " : 

 Quart. Journ. Geol. Soc, vol. Ixvi, p. 88, 1910. 



"^ Water-Supply of Lincolnshire (Mem. Geol. Surv.), 1904, pp. 33-5. 



^ H. Preston, " On a New Boring at Caythorpe " : Q.J.G.S., vol. lix, p. 29,, 

 1903. 



