Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/186

152 PROCEEDINGS OF THE GEOLOGICAL SOCIETY, [Feb. 7, or nearly flat-lying Secondary formations that now form great part of France and England (then united), were so far affected by this renewed upheaval of the Alps and Jura, that they were all tilted at low angles to the north-west. That circumstance gave the initial north-westerly direction to the flow of so many of the existing rivers of France, and led them to excavate the valleys in which they run, including the upper tributaries of the Loire and Seine, the Seine itself, the Marne, the Oise, and many more of smaller size ; and my surmise is, that this same westerly and north-westerly tilting of the chalk of England formed a gentle slope towards the mountains of Wales, and the rivers of the middle and south of England at that time flowed westerly. This first induced the Severn to take a southern course, between the hilly land of Wales and Herefordshire and the long slope of Chalk rising to the east ; and, aided by the tributary streams of Herefordshire, it cut a channel towards what afterwards became the Bristol Channel, and established the beginning of the escarpment of the chalk (e, fig. 4, p. 154), which has since gradually receded, chiefly by atmospheric waste, so far to the east. If this be so, then the origin of the valley of the Severn is of immediate post-Miocene date, and it is one of the oldest in England *.

The Avon, which is a tributary of the Severn, and joins it at Tewkesbury, is, at all events, partly of later date. It rises at the base of the escarpment of the Oolitic rocks east of Rugby, and gradually established its channel in the low grounds formed of Lower Lias and New Red Marl, as that escarpment retired eastward by virtue of that law of waste, that all inland escarpments retire opposite to the steep slope, and in the direction of the slope of the strata.

If the general slope of the surface of the chalk had been easterly instead of westerly at the post-Miocene date alluded to, then the initial course of the Severn would also have been easterly, like that of the Thames and the rivers that flow into the Wash and the Humber.

This at once leads to the question, Why is it that the Thames and other rivers that flow through the Oolites and Chalk run eastward? The answer seems to me to be, that after the original valley of the Severn was cut out by its river a new disturbance of the whole country took place, by which the Cretaceous and other strata were tilted eastward, and not suddenly, but by degrees, an initial slope was given to the Chalk and Eocene strata, east of the comparatively newly formed escarpment of the Chalk indicated by the dark line in fig. 4, marked e. The Chalk escarpment, in its beginning, is thus of older date than the Oolitic escarpment, though it would be hard to find this out except on the hypothesis I have stated. The Thames, then, in its beginning, from end to end, flowed easterly over Chalk and Eocene strata, and the river was larger then than now. But by processes of waste identical with those that formed the escarpment of the Wealden, the Chalk escarpment gradually


 * Many of the valleys of Wales may be much older.