GEOLOGY 



were identified by Mr. H. N. Dixon and Mr. Clement Reid : oak, 

 hazel (abundant), alder, common elder, pine (two species), bird-cherry, 

 blackthorn, dog's mercury, knot grass (?), yellow water-lily, chickweed. 

 The terrestrial animals found in the Alluvium include : ox (both Bos 

 taurus var. primigenius and Bos taurus var. longifrons), horse, sheep, wild 

 hog, red deer, etc. 



In the Alluvium wood is mostly quite black ; many of the bones, 

 smaller pieces of wood, and even stones are bright blue from the deposi- 

 tion on them of vivianite (phosphate of iron) ; water running from 

 the sandy layers leaves a red deposit. These effects may be explained 

 thus : The organic acids produced by decomposing vegetable matter 

 dissolve iron out of the ferruginous silt ; the solution of iron impreg- 

 nates the wood, and with the tannin there produces the deep black 

 colour ; the same solution reacting on the phosphates in the bones pro- 

 duces blue phosphate of iron ; and lastly, the soluble neutral crenates and 

 apocrenates of iron, on exposure to the air, turn into insoluble basic 

 ones, hence the red deposit from the water. 



The Windings of the Nene 



The windings of the Nene and other streams call for a few remarks. 

 A stream not perfectly straight to start with must of necessity get more 

 crooked, for every convex bend of one bank causes the water to impinge 

 on the opposite one, by which the latter is under-cut, and gradually 

 worked backwards into a deep curve with a vertical or even overhanging 

 face. One might expect the convex bank to exhibit the very opposite 

 characters, a very gentle long slope to the water ; so it does very 

 frequently ; still there are many places, more particularly in small valleys, 

 where, apparently without artificial aids, both banks are fairly perpendi- 

 cular. The reason appears to be found in a periodical expansion of the 

 clay soil and subsoil on absorption of water after drought, which expan- 

 sion causes the ground to creep in the direction of least pressure, i.e. the 

 stream. 



Minor Valleys and Springs 



What happened in the early stages of formation of the larger valleys 

 may now be observed in the smaller ones. It may be inferred from a 

 study of the map, but better still by field observation, that every valley 

 permanently or periodically carries a stream of water ; the valley and the 

 stream being (except in the older river valleys) intimately related in respect 

 of size. Again, in almost every case the bottom of the valley and parts of 

 the sides consist of impervious clay, and the higher parts of the bounding 

 hills of porous rock, and we will take as a typical example one where the 

 porous rock is Northampton Sand and the impervious one Upper Lias 

 Clay. 



The Northampton Sand is usually very porous throughout, and 

 when its junction with the underlying impervious clay was first exposed 

 by denudation, in the early stages of valley formation, water would run 



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