92 The Geology of the Railway Line from 



a most instructive model of the geology round London, and it I 

 should be visited and carefully studied by all those wishing to 

 understand the relations between underground and surface geology. 

 This model is on the scale of 6 inches to one mile, and is cut 

 through along various lines of section to show the subterranean 

 extent of the several formations. 



It is these railway cuttings as well as those along roads, together 

 with wells, quarries, brickyards, banks, and ditches, and even the 

 burrows of the humble rabbit and mole, that help the geologist to 

 make a geological map. 



The diagram begins a little north of Chiseldon Station and ends 

 at Collingbourne Station. It includes the upper cretaceous beds 

 from the gault to the upper chalk, and shows also the Kimeridge 

 clay. It is merely a diagram, and is not drawn to scale, and covers 

 in a small space a very great deal of ground. 



The line * — * marks the level of the rails. Above that you ; I 

 have the irregular line showing the top of the cutting ; then you 

 have the map attached to the cutting, which the cutting has 

 helped to make. If we look at the diagram we shall see that the 

 beds by Chiseldon have a slope, or " dip " as it is called, to the 

 south, and that this south dip continues till you pass Marlborough, 

 where the direction of the dip changes and the beds are also bent 

 up so that at Savernake they dip north, while at Graf ton they again 

 dip south. 



Below the level of the line * — * the beds have been continued 

 below the surface of the ground to show their order underground, 

 and the effect of the uprise, forming the Pewsey Yale. 



The beds in our cutting fall into three classes, clays, sands, and 

 limestones, represented by the gault, the upper greensand, and the 

 chalk. 



There is a gradual passage from the gault clay to the upper 

 greensand, the clay growing more sandy till it becomes a true sand, 

 and again there is a passage from the greensand to the chalk, the 

 former becoming more and more calcareous till it passes into chalk. I 

 But in the lower part of the lower chalk are some very 

 sandy and gritty beds, the middle chalk differs from the upper 



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