802 REPORT—1896. 
officers of the Geological Survey, the late Professor Carvill Lewis, Mr. Lamplugh, 
and other geologists, to the action of glaciers descending all the principal valleys, 
from Teesdale on the north to Airedale on the south, with a great main stream 
occupying the Vale of York almost as far as the Humber, and a Scandinavian 
ice-sheet abutting against the whole coast-line. 
Pre-Glacial valleys have been detected beneath the drift at depths exceeding 
170 feet below O.D. 
The irregular accumulation of the glacial deposits produced many changes in 
the courses of the rivers, and a great area was added to the coast-line. 
The Derwent has been shown by Mr. Fox Strangways to have reversed its 
flow, and instead of discharging into Filey Bay, it now flows westward, passes 
through the Howardian Hills in a narrow gorge 150 feet deep, and ultimately 
joins the Ouse. The change of direction has been ascribed to the formation of a 
ridge of boulder-clay, which extends across the valley behind Filey, and has a 
minimum altitude of 130 feet, which is only 70 feet below the top of the notch in 
the Howardian Hills. The author considers it more probable that the diversion 
was effected by an ice-barrier. At one-stage a lake would be formed occupying 
the whole Vale of Pickering, and lacustrine deposits are found, having a thick- 
ness of over 90 feet. 
The river system of the Vale of York is very peculiar. The Tees crosses a very 
broad tract of soft rocks without receiving a single tributary from the south. The 
Wiske rises in the north-western corner of the Cleveland Hills, and approaches 
within two miles of the Tees, then turns south and joins the Swale. 
The Drift is very deep along the line of the Tees, and thins to the south, so 
that the solid rocks are exposed at many places along a line running through 
Northallerton and Bedale. ‘This was the pre-Glacial Watershed. Northward of 
it the Drift is mainly boulder-clay, while southward gravels largely predominate ; 
exactly the same fact is observed south of the watershed between the Mersey and 
the Severn. 
The Swale and Wiske were formerly tributaries of the Tees. 
No study has yet been made of the Ure. 
The Mdd furnishes an example of a diversion different from any yet noted. 
Down to Ripley it flows through a wide and open valley, but below that village it 
enters a narrow and deep gorge or ravine cut partly through grits and shales of 
the Millstone Grit series, and partly through Magnesian Limestone. For long 
distances its banks are extremely steep, and in places, as at Knaresborough and 
Plumpton, even vertical, producing scenery unrivalled in any part of Yorkshire. 
This 1s obviously so recent a channel that the author was impelled to seek an 
older one, and discover the cause of its abandonment. Such an old valley is 
clearly traceable from Ripley, past Nidd Hall and Brearton, out into the Vale of 
York. It is broad and well defined, and its sides have a very gentle slope, like 
those of the upper part of the valley, and there are extensive marshy patches in its 
course. Near Nidd Hall a large lateral moraine of a glacier, which came down 
Uredale, obstructs the old valley. Many excavations display the usual structure 
of moraines. 
The Wharfe presents similar features to those of the Nidd. Its valley is wide 
and open until the town of Wetherby is reached; then the river, instead of pur- 
suing a north-easterly course through a valley extending through the town, 
turns abruptly to the south-east, and runs through a gorge in the Magnesian 
Limestone down to Tadcaster. The valley across the site of Wetherby is filled 
with a great thickness of excessively coarse morainic gravel, thrown down by the 
side of the same glacier as that which deflected the Nidd, and it seems probable 
that this also is a case of diversion. 
There are numerous small diversions of the Aire by terminal moraines—for 
example, near Keighley and Bingley—but its lower course appears quite normal. 
Great changes have been wrought in the upper part of the Calderdale by the 
events of the Glacial period, but they and the remarkable vicissitudes of the 
Trent will be dealt with in a future communication. 
