part 3] THE PLEISTOCENE DEPOSITS A.HOUND CAMBHIDGE. 2U> 



a set of ferruginous 'pockets' ending abruptly against the base of 

 the overlying sands and gravels, and easily distinguishable in their 



character from the higher ' pipes " which abut against the subsoil : 

 these are shown in the appended section, fig. 2. The material in 

 the lower 'pockets' is reddish brown, similar in character to, and 

 probably of the same age as, the loam in the subterranean channels. 

 The loam is different from the sand and gravel which occur at 

 the sides and bases, and these loamy masses seem to be folded 

 into the subjacent deposits, as indicated by contortion of the latter 

 a- seen in tig. 2. 



The evenly-bedded deposits above these loamy masses occur 

 about the top of the ridge between the Cam Valley and the small 

 valley south- wot of the Observatory. 



In October 15)19 one of the loam-filled subterranean erosion- 

 channels was also found to be cut off above by the same sands and 

 gravels, as shown in the same section ; it would appear, therefore, 

 that some time elapsed, after the formation of the unevenly-bedded 

 gravels and the loam now tilling the channels and 'pockets,' before 

 highest sands and gravels were laid down. 



We find, then, four distinct sets of deposits here, namely : the 

 lower evenly-bedded sands and gravels, the unevenly-bedded sands 

 and gravels, the loams in the channels, and the uppermost thin 

 regularly-bedded sands and gravels, only recently exposed as the 

 pit is being cut backward. 



The lower evenly-bedded series consists of deposits of various 

 degrees of coarseness. The base always shows a fairly coarse 

 gravel, with a very clayey matrix derived from the Gault below. 

 Above are alternating gravels of various degrees of coarsen.- v. 

 sands, and light-coloured loams. Big boulders are found in these 

 beds, and. according to the foreman of the pit, chiefly in the basal 

 deposit. 



The unevenly-bedded gravels consist of materials very similar to 

 those of the underlying deposits, apart from the irregularity of their 

 stratification, save that massive deposits of gravel with a loamy 

 matrix devoid of stratification are frequent. Lenticular deposits 

 of finely-bedded sand occur at various horizons in the upper a 

 usually tilling small erosion-channels and showing false bedding 

 parallel to the floors of the channels. Many boulders also occur in 

 the unevenly-bedded series, especially in its basal deposit, which is 

 usually a coarse ferruginous gravel. 



The third series of deposits — the loams — are chocolate-brown 

 sandy loams with scattered flint-pebbles. Seams of gravel occur 

 in them occasionally. Their occurrence iii ordinary erosion- 

 channels is of importance, as indicating that they were deposited 

 not long after the deposition of the uppermost layers of the 

 unevenly-bedded gravels. Their occurrence in the subterranean 

 erosion-channels is of interest, as indicating that they must 

 have been laid down over a wider area than that in which they 

 now occur, but have escaped erosion where let down into these 

 channels. That the channels are due to subterranean erosion is 



