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 31
