SUCCESSION IN THE TEENT BASIN. 473 



of the furrows aud ridges of the contortions, or the direction in 

 which the underlying rock has been trailed. 



The Later Pennine Boulder-clay and contortions of the same age 

 are scattered over the greater part of the Trent basin, and attain 

 their greatest development in the iuterglacial valleys, or on the more 

 elevated plateau-like areas. In postglacial times the clay was 

 carried away from large districts by river-action, or denuded from 

 the steeper slopes by ordinary atmospheric influences ; it is now 

 found where the land is tolerably flat, and might be passed over in 

 the absence of boulders, or when the rocks beneath it are tolerably 

 uniform in character or show no very evident traces of bedding. 

 The absence of pebbly or sandy beds in the neighbourhood immedi- 

 ately to the north or west also contributes much to this result. 



Many geologists express themselves very doubtful as to whether 

 ice-action has really in any way contributed to the formation of the 

 contortions and masses of unstratified pebbly clay, preferring to 

 attribute their existence to a slow soil-cap motion or the disturbing 

 action of roots. IS'o doubt the feeling arises from a knowledge that 

 it is always unsafe to attribute the existence of any phenomenon to 

 the action of an agency which is little understood. I have kept 

 this well in view, but after a careful examination of many hundreds 

 of sections, I see no reason whatever to doubt the glacial origin of 

 the contortions and stony clays. 



Of the reasons which have led me to this view, the following are 

 among the more important : — 



1st. The trend of the ridges and furrows of the contortions 

 seldom shows any dependence whatever upon the direction of the 

 slope. In many instances the action has been down the valley at 

 right angles to the slope of its sides. 



2nd. In some cases a movement up the slope is indicated. 



3rd. The contortions are equally well developed on flat areas. 



4th. On the steeper slopes formed by postglacial erosion contor- 

 tions are quite absent, the only effect of the angle and exposure 

 being to slightly dip the beds towards the valley. 



5th. When the contorting force has faulted gravels, the quartz, 

 quartzite, and flint pebbles along the line of disturbance are fre- 

 quently fractured, the fragments remaining in close juxtaposition. 



6th. The displacement and breaking up of massive rock-beds also 

 points to the action of a powerful agency. 



7th. The contortions are exactly similar to those beneath the 

 older Boulder-clays. 



8th. The direction of the ice-flow, as indicated by the trend of the 

 ridges and furrows, is corroborated by the origin of the included 

 erratics. 



9th. Well-striated erratic boulders sometimes occur in the surface 

 Boulder-clays. 



10th. All traces of contortion are absent from postglacial river- 

 gravels, even where they occupy terraces as much as fifteen feet 

 above the valley-bottoms, the terrace shown in fig. 3 always 

 separating the newer from the older contorted gravels. 



