1872.] RAMSAY RIVER-COURSES OF ENGLAND AND WALES. 153
receded eastward; and as it did this the area of drainage contracted.
By and by the outcropping edges of Oolitic strata became exposed,
and a second and later escarpment began to be formed ; but the
escarpment of the Chalk being more easily wasted than that of the
Oolite, its recession was more rapid.
All this time the Thames was cutting a valley for itself in the Chalk ; and by and by, when the escarpment had receded to a certain point, its base was lower than the edge of the Oolitic escarpment that then as now overlooked the valley of the Severn ; only at that time the vally was narrower. While this point was being reached, the Thames by degrees was joined by the waters that drained part of the surface of the long eastward slope of the Oolitic strata, the western escarpment of which was still receding ; and thus was brought about what at first sight seems the unnatural breaking of the river through the high escarpment of the Chalk between Wallingford and Reading. This, also, is the reason why the so-called sources of the Thames, the Seven Springs and others, rise so close to the great escarpment of the Inferior Oolite east of Gloucester and Cheltenham. But the sources of the river now are not more stationary than those that preceded them. The escarpments both of Chalk and Oolite are still slowly changing and receding eastward, and as that of the Oolite recedes, the area of drainage will diminish and the Thames decrease in volume. This is a geological fact, however distant it may appear to persons unaccustomed to deal with geological time.
The same kind of argument is applicable to the Ouse, the Nen, the Welland, the Glen, and the Witham, rivers flowing into the Wash, all of which rise either on or close to the escarpment of the Oolites, between the country near Buckingham and that east of Grantham, on rocks which were once covered by the Chalk.
With minor differences, the same general theory equally applies to all the rivers that run into the Humber. I believe the early course of the Trent was established at a time when, to say the least, the Lias and Oolites overspread all the undulating plains of New Red Marl and Sandstone of the centre of England, and passed out to what is now the sea, beyond the estuaries of the Mersey and the Dee. A high-lying anticlinal line threw off those strata, with low dips to the east and west ; and, after much denudation, the large outlier of Lias between Market Drayton and Whitchurch, in Shropshire, is one of the western results. Down the eastern slopes the Trent began to run across an inclined plain of Oolitic strata. Through long ages of waste and decay the Lias and Oolites were washed away from all these midland districts, and the long escarpment formed of these strata now lies well to the east, overlooking the broad valley of New Red Marl through which the river flows.
The most important tributary of the Trent is the Derwent, a tributary of which is the Wye of Derbyshire. The geological history of the latter river is very instructive. It runs right across part of the central watershed of England formed by the great boss of the Carboniferous Limestone of Derbyshire. This course, at first sight,
