part 3] PLEISTOCENE DEPOSITS AROUND CAMBRIDGE. 207 



Philosophical] Society I arranged the deposits, so far as I had then 



studied them, in the following order : — 



Barnwell -Station Beds. 

 Upper Barnwell- Village Be<ls. 

 Observatory Beds. 

 Lower Barn well- Village Beds. 



Since then other deposits have been studied, but those enumerated 

 in that paper seem to he the most important. 



It will be convenient to describe the deposits in the order of 

 what I believe to be their relative ages, beginning with the oldest. 

 The general distribution of the deposits will be made clear by the 

 accompanying sections, one of which is from ElHeda House, a mile 

 east of Barnwell, across the Observatory Ridge to the Madingley 

 Ivoad (PI. XT, iig. 1), and the other from Barnwell Station to 

 Newnham (PI. XI, fig. 2). 



(a) The Corbicula Beds — Lower Barnwell- Village Beds. 



The occurrence of CorhicuJd in various places around Cambridge 

 has been recorded in previous writings, in deposits which the 

 Geological Surveyors refer to the Highest Cam Terrace of the 

 gravels of the present river-system, though I hope to show that 

 deposits of different ages belong to this 'terrace/ In the immediate 

 vicinity of Cambridge the Corbicula-he&ring beds arc found at a 

 height of 20 to 4-3 feet above sea-level. They are usually covered 

 by gravels and sands which appear to be of later date, though at 

 the Rodney Brewery, Barnwell, they apparently occur immediately 

 below the surface-soil, as shown in the section given by the late 

 Prof. Hughes, in his paper on the gravels of East Anglia, fig. 31, 

 p. 45. In addition to this place, Corbicula has been found 

 near Barnwell Abbey, the old locality, long since covered; at the 

 Milton-Road gravel-pits, immediately north of the junction of that 

 road with the Chesterton Road ; and in a section to be presently 

 described, in the Museum grounds on the Downing Site. It is 

 interesting to note that the beds on this site are referred by the 

 Surveyors to the ' Intermediate Terrace 1 and not to the 'Highest 

 Terrace.' The deposits consist of gravel, sands, and loams, the 

 molluscs being usually found in a pale buff-coloured sandy loam. 

 From the Barnwell- Abbey and Milton-Road Pits a rich mamma- 

 lian fauna is recorded ; but the fossils were chiefly derived from 

 the beds in the upper parts of the pits, whereas the Corbicula 

 came from beds at the base which were not often exposed. As 

 the deposits lie at about tin 1 same level over a wide area, they were 

 probably formed on an extensive alluvial tract, and some mav even 

 be lacustrine. 



In trying to determine the age of these beds, one must discus-* 

 the palseontological evidence ; for there seems to be a conflict of 

 opinion in this country as to the age of the Pleistocene C&rbiculm. 



A very significant feature is the frequent association here of 



