, now narrowing
into gorges, now opening out into wide plains, and again narrowed lower
down; and it is just in these openings in the valleys that the "till" is
said to be found, and also in the lowlands where an ice-sheet must have
extended for many miles in every direction. In these lowland valleys the
"till" is both thickest and most wide-spread, and this is what we might
expect. At first, when the glaciers from the mountains pushed out into
these valleys, they would grind out the surface beneath them into hollows,
and the drainage-water would carry away the _debris_. But when they spread
all over the surface from sea to sea, and there was little or no drainage
water compared to the enormous area covered with ice, the great bulk of the
_debris_ must have gathered under the ice wherever the pressure was least,
and the ice would necessarily rise as it accumulated. Some of the mud would
no doubt be forced out along lines of least resistance to the sea, but the
friction of the stone-charged "till" would be so enormous that it would be
impossible for any large part of it to be disposed of in this way.

[36] That the ice-sheet was continuous from Scotland to Ireland is proved
by the glacial phenomena in the Isle of Man, where "till" similar to that
in Scotland abounds, and rocks are found in it which must have come from
Cumberland and Scotland, as well as from the north of Ireland. This would
show that glaciers from each of these districts reached the Isle of Man,
where they met and flowed southwards down the Irish Sea. Ice-marks are
traced over the tops of the mountains which are nearly 2,000 feet high.
(See _A Sketch of the Geology of the Isle of Man_, by John Horne, F.G.S.
_Trans. of the Edin. Geol. Soc._ Vol. II. pt. 3, 1874.)

[37] _The Great Ice Age_, p. 177.

[38] These are named, in descending order, Hessle Boulder Clay, Purple
Boulder Clay, Chalky Boulder Clay, and Lower Boulder Clay--below which is
the Norwich Crag.

[39] "On the Climate of the Post-Glacial Period." _Geological Magazine_,
1872, pp. 158, 160.

[40] _Geological Magazine_, 1876, p. 396.

[41] _Early Man in Britain and his Place in the Tertiary Period_, p. 113.

[42] Heer's _Primaeval World of Switzerland_ Vol. II., pp. 148-168.

[43] Dr. James Geikie in _Geological Magazine_, 1878, p. 77.

[44] This subject is admirably discussed in Professor Asa Gray's Lecture on
"Forest Geography and Archaeology" in the _American Journal of Science and
Ar