of any contradiction; but although geologists agree as to its existence,
they do not find it easy to absolutely determine its date or its causes.

Croll's theory of the eccentricity of the Earth's orbit[1] as the chief
factor in the great changes of the Earth's climate has now been to a great
extent abandoned, and the approximate date of the Glacial Epoch of between
240,000 and 80,000 years ago is thus correspondingly discredited by many
geologists. Professor Kendall inclines to the belief that not more than
25,000 years have elapsed since the departure of the ice from Yorkshire,
the freshness of all the traces of glaciation being incompatible with a
long period of post-glacial time.

[Footnote 1: "Climate and Time." James Croll, 1889.]

The superficial alterations in the appearance of these parts of Yorkshire
were brought about by the huge glaciers which, at that time, choked up
most of the valleys and spread themselves over the watersheds of the land.

In the warmer seasons of the year, when the Arctic cold relaxed to some
extent, fierce torrents would rush down every available depression,
sweeping along great quantities of detritus and boulders sawn off and
carried sometimes for great distances by the slow-moving glaciers. The
grinding, tearing and cannonading of these streams cut out courses 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