GENERAL PRINCIPLES. 
119 
tinent, and even to Greenland, were presently replaced by the artic rigor 
of the Greenland of to-day, the vast ice sheet which caps the polar regions 
having pushed its margin down to the latitude of southern Pennsylvania 
and Ohio, and along the higher Appalachian valleys to the very borders 
of the Southern States. This change was accompanied, and no doubt 
largely, if not mainly produced by great changes in the topography of 
the continent, the chief of which was doubtless an elevation of the sub- 
polar regions and a great increase of frozen land surface. One conse- 
cpience of these vast changes was the extinction of nearly all the pre-exist- 
ing races of plants and animals. The formations accumulated during 
this period, known as the Glacial or Ice Period, are frequently called 
Post-pliocene and Post Tertiary , but more commonly Quaternary ; and 
since it has been ascertained that they were formed mainly by the action 
of ice, they are called Drift or Glacial Drift. They consist of beds of 
pebbles, gravel and sand, and of clay mingled with stones, called bowlder 
clay, and scatted rock masses, more or less rounded, called bowlders, often 
of great size. These materials were abraded by the great ice masses in 
their southward movement, from the underlying rocks, and were either 
left in confused heaps, when the ice melted, or have been partially ar- 
ranged in more or less nearly horizontal beds, — stratified by the subsequent 
action of water. These accumulations of course contain no marine 
fossils, and the remains of land animals and even plants are seldom abun- 
dant. The stratified drift is found especially in terraces or benches 
flanking the large streams or in areas which were occupied by lakes or 
broad estuaries. 
From the above account of the origin of these formations, it is evident 
that they will be found mostly in high latitudes, (or in elevated and 
mountainous regions further south) ; but in the Middle and even South- 
ern States are found Quarternary beds of pebbles, gravel, sand and clay, 
large tracts of these regions having been submerged during at least a part 
of the glacial period and covered with the material swept down from ad- 
jacent highlands and from the more northern zone by the great floods to 
which the melting ice gave rise. 
Beds of drift abound in the New England and northern states ; and in 
the Mississippi valley quite to the Gulf, are found great areas of stratified 
Quaternary sands, clays, &c., derived from more northern sources. On 
the Atlantic slope also, in Virginia and North Carolina, beds of a similar 
character and age extend inland more than 100 miles from the coast. 
Quaternary beds are very abundant in England and among the Alps, 
as well as elsewhere on the continent, and in all high latitudes of both 
hemispheres. 
