318 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Juiie 15, 



Great Oolite, a series of shales, sands, and clays (the equivalent of 

 the Upper Sandstone and Shale of Yorkshire), Cornbrash, Kelloway 

 Rock, and Oxford Clay. Covering some portions of these, and in 

 considerable thickness, is a great mass of the northern drift, besides 

 vphich are thick accumulations of gravel and remains of a fluviatile 

 deposit. 



The beds referable to the Drift period which it is intended to 

 describe in the first part of this paper, viz. the superficial detritus 

 occurring in irregular patches on the oolite, the boulder-clay or 

 northern drift, and the gravel beds of the valleys, &c., will be noticed 

 as they occur from north to south, commencing from near Grantham. 



Part I. The Drift. 



Section 1. Little Ponton Cutting. — The first indications, in these 

 sections, of deposits connected with the Drift era (and probably towards 

 the termination of it) are met with at the Ponton Cutting, south of 

 the Witham valley, where the oolites are frequently dislocated, the dis- 

 located portions lying at high angles and very irregular. The oolitic 

 rocks, moreover, are here scooped out into hollows of considerable 

 size, sometimes 100 yards in length and 50 feet in depth : these 

 cavities are extremely irregular in form and have a general direction 

 of N.E. and S.W., and apparentl)^ follow the direction of the great 

 jointings or fissures of the rock. They are occupied by more or less 

 stratified masses of clays, and sands with pebbles, with occasional 

 rounded boulders of sandstone. Interstratified with these, but more 

 especially towards the upper portion, are thick layers of fragmentary 

 oolite, identical with the enclosing rock, and sometimes, in the lower 

 part, large blocks of the same rock. 



Section 2. Between the two Ponton Cuttings. — The first appear- 

 ance of the northern drift is met with in a small section, about 

 6 feet deep and about 160 yards in length, between the two Pon- 

 ton Cuttings. The drift here overlies the upper part (shell)! beds) 

 of the Great Oolite. 



Section 3. Great Ponton Cutting. — The surface of the oolite is 

 here excavated by similar hollows to those described in the Little 

 Ponton Cutting, and these are also occupied by a similar debris which 

 is distinct from and probably posterior to the northern drift. 



At the southern end of this cutting the drift is again met vdth. 

 Here a patch of the drift is exposed, 380 yards in length at its upper 

 part and 160 yards at the base (the depth of the cutting being 

 20 feet), and overlying the shelly beds of the Oolite, which here dip 

 to the S. at a moderate angle (1, 2, 3 in fig. I). Passing a deeply- 

 denuded valley in the drift, we come to a hill composed of the same 

 formation, through which a tunnel is excavated (see fig. 1). The 

 summit elevation of this hill of drift above the datum line is about 

 500 feet, and the drift here forms nearly the highest ground of the 

 district. At the northern extremity of the tunnel the drift appears 

 to be di^dsible into two portions. The upper part, about 25 feet 

 thick, consists of light grey sandy clay, full of angular and rounded 

 flints, some 2 feet in diameter, and rolled fragments of chalk, vary- 



