166 
ELEMENTS OE GEOLOGY. 
CHAPTER XL 
POST-PLIOCENE PEEIOD, CONTINUED,—GLACIAL CONDITIONS.'*' 
Geographical Distribution, Eorai, and Characters of Glacial Drift.—Funda¬ 
mental Rocks, polished, grooved, and scratched.—Abrading and striating 
Action of Glaciers.—Moraines, Erratic Blocks, and “ Roches Moutonnees.” 
■—Alpine Blocks on the Jura.—Continental Ice of Greenland.—Ancient 
Centres of the Dispersion of Erratics.—Transportation of Drift by floating 
Icebergs.—Bed of the Sea furrowed and polished by the running aground 
of floating Ice-islands. 
Character and Distribution of Glacial Drift.— In speaking of 
the loose transjiorted matter commonly found on the surface 
of the land in all parts of the globe, I alluded to the excep¬ 
tional character of what has been called the boulder forma¬ 
tion in the temperate and Arctic latitudes of the northern 
hemisphere. The peculiarity of its form in Europe north of 
the 50th, and in North America north of the 40th parallel of 
latitude, is now universally attributed to the action of ice, 
and the difference of opinion respecting it is now chiefly re¬ 
stricted to the question whether land-ice or floating icebergs 
have played the chief part in its distribution. It is wanting 
in the warmer and equatorial regions, and reappears when 
we examine the lands which lie south of the 40th and 50th 
parallels in the southern hemisphere, as, for example, in Pata¬ 
gonia, Terra del Fuego, and New Zealand. It consists of 
sand and clay, sometimes stratified, but often wholly devoid 
of stratification for a depth of 50,100, or even a greater num¬ 
ber of feet. To this unstratified form of the deposit the name 
of till has long been applied in Scotland. It generally con¬ 
tains a mixture of angular and rounded fragments of rock, 
some of large size, having occasionally one or more of their 
sides flattened and smoothed, or even highly polished. The 
smoothed surfaces usually exhibit many scratches parallel to 
each other, one set of which often crosses an older set. The 
till is almost everywhere wholly devoid of organic remains, 
except those washed into it from older formations, though in 
some places it contains marine shells, usually of northern or 
* As to the former excess of cold, whether brought about by modifications 
in the height and distribution of the land or by altered astronomical condi¬ 
tions, see Principles, vol. i. (10th ed., 1867), chaps, xii. and xiii., “ Vicissi¬ 
tudes of Climate.” 
