116 



A. J. JUKES-BROWJNE ON THE 



I propose, therefore, to set down some of the evidence which I 

 have obtained during the last six years, in order that the present 

 state of the case may be properly apprehended, and that the links 

 which are wanting in the chain of evidence, and the difficulties which 

 call for explanation, may be clearly perceived. 



JSTo one who studies the Lincolnshire Boulder-clays can fail to be 

 struck with the great differences between the two principal types of 

 glacial clay which occur in the county ; they fall naturally into two 

 groups — a brown series and a grey or blue series, which are not 

 only lithologically different, but also to a great extent geographically 

 separate. It will perhaps clear the ground if I commence by briefly 

 sketching the range of these two types of Boulder-clay, leaving their 

 points of contact to be considered afterwards. The relations of the 

 two members of the " brown series," namely the Purple and Hessle 

 Clays, will next be discussed ; and in conclusion such inferences will 

 be drawn as appear to be warranted by the facts described *. 



§ 1. The JRange or superficial Extension of the ChalTcy Boulder -day . 



This type of Boulder- clay only occurs in the southern, central, and 

 western parts of the county. In the south-west, about Corby and 

 Ponton, it is generally of a light grey or greyish-blue colour full of 

 chalk and oolitic debris ; beneath the Pens and northward along 

 the central valley it is usually of a deep blue or blue-grey ; but 

 eastward as it nears the Chalk Wolds it becomes lighter and lighter, 

 finally passing into an intensely chalky clay or marl of a pure white 

 or yellowish- white tint. 



There can, I think, be little doubt that these grey-blue and white 

 Boulder-clays form part of the great sheet which spreads over the 

 eastern Midlands, and which appears to be continuous with what has 

 been termed by Mr. Searles "Wood the Chalky Boulder-clay or Upper 

 Boulder- clay of East Anglia. 



The mapping of sheets 70 and 83 1 has disclosed certain peculi- 

 arities in the lie of this Boulder-clay which deserve attention. In 

 sheet 64 (south of sheet 70) spreads of Boulder-clay occur both on 

 the lowest and on the highest levels ; and some of these entering 

 sheet 70 extend northward along the high ground by Great Ponton 

 and Somerby to Kelby, a village about two miles S.E. of Ancaster. 

 But from Ancaster and Sleaford northward to Lincoln no trace of 

 this clay has been found along any part of the tract occupied by the 

 outcrops of the Jurassic strata. Again from Lincoln northward 

 no Boulder-clay is found along the higher ground, though it is 



* For a full description of the Lincolnshire Boulder-clays and of the sections 

 observed during the progress of the Geological Survey the reader is referred to 

 the explanations of sheets 70, 83, 84, 85, and 86, vphich are now in course of 

 publication. In the following pages only a few sections are described as 

 specially illustrating the relations of the several clays, and the paper is designed 

 to convey such theoretical conclusions as do not find a place in the Survey 

 Memoirs. These conclusions are not, however, to be taken as expressing the 

 general opinion of the officers of the Survey, but are my own personal views. 



t These numbers refer to the sheets of the Ordnance Survey map, and their 

 limits are shown on the map accompanying this paper. 



