Maw — On Subaerial and Marine Denudation. 



443 



roughly reproduced on the surface of the overlying deposit. This 

 following of the irregular basement line, in superimposed heds, is 

 well shown in Figure 2, representing a section on the Bristol and 



Fig. 2. — Section neae, the Bourton Station on the Bristol and Exeter 



Eailway. 



Exeter Eailway, near the Bourton station, six miles from Bristol, 

 where the Keuper Marls dip away in each direction at an angle of 

 from 4° to 6° from the bosses of Mountain Limestone over which 

 they were deposited. It seems probable that the main valleys and 

 lines of waterflow of the ancient buried surface would impress their 

 outline through a considerable thickness of superincumbent deposits 

 and thus determine the direction of the principal watercourses on 

 the new surface ; occasionally the new watercourses would be di- 

 verted from the ancient buried lines of waterflow by adventitious 

 irregularities of surface, especially in the case of the shallower 

 valleys, and it is in these that many of the cross cut valleys and trans- 

 verse gorges appear to have been initiated. 



Eeferring to Figures 3, 4, and 5, representing a series of valleys 



Fig. 3.- 



-Transverse Section of Ancient Valley System, 

 Section of Superimposed System, 



Longitudinal 



superimposed in part uncomformahly over an ancient buried series, 

 let us suppose the superincumbent deposit a a to be gradually de- 

 nuded by subaerial agency, the main lines of waterflow will simply 

 re-excavate and perhaps somewhat alter the old buried valleys over 

 which they are conformable, hut loherever the upper system of valleys c c 

 •runs transv&rsely (as in Fig. 3) to the buried system the separating 



