ourses for
themselves wherever they went. In some cases the stream would occupy an
existing hollow or old water-course, deepening and widening it, but in
many instances where the ice blocked a valley the water would form lakes
along the edge of the glacier, and overflowing across a succession of hill
shoulders, would cut deep notches on the rocky slopes.

Owing to the careful work of Mr C.E. Fox-Strangways and of Professor Percy
F. Kendall, we are able to tell, almost down to details, what took place
in the Vale of Pickering and on the adjacent hills during this period.

In the map reproduced here we can see the limits of the ice during the
period of its greatest extension. The great ice-sheet of the North Sea had
jammed itself along the Yorkshire coast, covering the lower hills with
glaciers, thus preventing the natural drainage of the ice-free country
inland. The Derwent carrying off the water from some of these hills found
its outlet gradually blocked by the advancing lobe of a glacier, and the
water having accumulated into a lake (named after Hackness in the map),
overflowed along the edge of the ice into the broad alluvial plain now
called the Vale of Pickering. Up to a considerable height, probably about
200 feet, the drainage of the Derwent and the other streams flowing into
the Vale was imprisoned, and thus Pickering Lake was formed.

The boulder clay at the seaward end of the Vale seems to have been capped
by ice of a thickness of nearly 100 feet which efficiently contained the
waters of the lake until they overflowed through a depression among the
hills to the south of Malton. If the waters escaped by any other outlet to
the west near Gilling and Coxwold, it can scarcely have been more than a
temporary affair compared to the overflow that produced the gorge at
Kirkham Abbey, as the Gilling Gap was itself closed by the great glacier
descending the Vale of York. The overflow of the lake by this route, south
of Malton, must have worn a channel down to a lower level than 130 feet
O.D. before the ice retreated from the seaward end of the Vale, otherwise
the escape would have taken place over the low hills blocking the valley
in that direction and the normal course of the drainage of the country
would have been resumed. The southern overflow evidently dug its way
through the hills fast enough to maintain that outlet, and at the present
time the narrow gorge at Kirkham Abbey is only 50 feet above sea level,
and the