CHAPTER X.

THE DATE OF THE GLACIAL PERIOD.

IN approaching the subject of glacial chronology, we are compelled to recognise at the outset the approximate character of all our calculations. Still, we shall find that there are pretty well-defined limits of time beyond which it is not reasonable to place the date of the close of the Glacial period; and, where exact figures cannot be determined, it may yet be of great interest and importance to know something near the limits within which our speculations must range.

For many years past Mr. Croll's astronomical theory as to the cause of the Glacial period has been considered in certain circles as so nearly established that it has been adopted by them as a chronological table in which to insert a series of supposed successive Glacial epochs which are thought to have characterised not merely the Quaternary epoch but all preceding geological eras. What we have already said, however, respecting the weakness of Mr. Croll's theory is probably sufficient to discredit it as a chronological apparatus. We will therefore turn immediately to the more tangible evidences bearing upon the subject.

The data directly relating to the length of time which separates the present from the Glacial period are mainly connected with two classes of facts:

1. The amount of erosion which has been accomplished by the river systems since the Glacial period; and 2. The

amount of sedimentation which has taken place in lakes
and kettle-holes. We will consider first the evidence
from erosion.

The gorge below Niagara Falls affords an important
chronometer for measuring the time which has elapsed

THOUSAND YEARS AGO FROM FIG. 103.-Diagram of eccentricity and precession: Absciss represents time and ordinates, degrees of eccentricity and also of cold. The dark and light shades show the warmer and colder winters, and therefore indicate each 10,500 years, the whole representing a period of 300,000 years.

since a certain stage in the recession of the great North
American ice-sheet. As already shown, the present Niag-
ara River is purely a post-glacial line of drainage; * the
preglacial outlet to Lake Erie having been filled up by
glacial deposits, so that, on the recession of the ice, the
lowest level between Lake Erie and Lake Ontario was in
the line of the trough of the present outlet. But, from
what has already been said, it also appears that the Niagara
River did not begin to flow until considerably after the
ice-front had withdrawn from the escarpment at Queens-
ton, where the river now emerges from its cañon to the
low shelf which borders Lake Ontario. For a considerable
period afterwards the ice continued to block up the east-
erly and northerly outlets through the valleys of the

SCALE OF MILES

0

Niagara

Falls of Niagara

COAT 1.

E

Struthers & Co., Engr's, N. Y.

FIG. 104.-Map of the Niagara River below the falls, showing the buried channel from the whirlpool to St. Davids. Small streams, a, b, c, fall into the main gorge over a rocky escarpment. No rock appears in the channel at d, but the rocky escarpment reappears at e.

NIAGARA

RIVER

W

Y O

Ꭱ

K

Mohawk and of the St. Lawrence, and held the water in front of the ice up to the level of the passes leading into the Mississippi Valley. Niagara River, of course, was not born until these ice-barriers on the east and northeast melted away sufficiently to allow the drainage to take its natural course.

Of these barriers, that across the Mohawk Valley doubtless gave way first. This would allow the confluent waters of this great glacial lake to fall down to the level of the old outlet from the basin of Lake Ontario into the Mohawk Valley, in the vicinity of Rome, N. Y. The moment, however, that the water had fallen to this level, the plunging torrents of Niagara would begin their work; and the gorge extending from Queenston up to the present falls is the work done by this great river since that point of time in the Glacial period when the ice-barrier across the Mohawk Valley broke away.

The problem is therefore a simple one. Considering the length of this gorge as the dividend, the object is to find the rate of annual recession; this will be the divisor. The quotient will be the number of years which have elapsed since the ice first melted away from the Mohawk Valley. We are favoured in our calculation by the simplicity of the geologic arrangement.

The strata at Niagara dip slightly to the south, but not enough to make any serious disturbance in the problem. That at the surface, over which the water now plunges, consists of hard limestone, seventy or eighty feet in thickness, and this is continuous from the falls to the face of the escarpment at Queenston, where the river emerges from the gorge. Immediately underneath this hard superficial stratum there is a stratum of soft rock, of about the same thickness, which disintegrates readily. As a consequence, the plunging water continually undermines the hard stratum at the surface, and prepares the way for it to fall down, from time to time, in huge blocks,

ONTARIO

LAKE

which are, in turn, ground to powder by the constant commotion in which they are kept, and thus the channel is cleared of débris.

Below these two main strata there is considerable variation in the hardness of the rock, as shown in the accompanying diagram, where 3 and 5 are hard strata separated by a soft stratum. In view of this fact it seems probable that, for a considerable period in the early part of the recession, instead of there being simply one, there was a succession of cataracts, as the water unequally wore back through the harder strata, numbered 5, 3, and 1;

FIG. 105.-Section of strata along the Niagara gorge from the falls to the lake: 1, 3, strata of hard rock; 2, 4, of soft rock.

but, after having receded half the distance, these would cease to be disturbing influences, and the problem is thus really the simple one of the recession through the strata numbered 1 and 2, which are continuous. So uniform in consistency are these throughout the whole distance, that the rate of recession could never have been less than it is now. We come, therefore, to the question of the rapidity with which the falls are now receding.

In 1841 Sir Charles Lyell and Professor James Hall (the State Geologist of New York) visited the falls together, and estimated that the rate of recession could not be greater than one foot a year, which would make the time required about thirty-five thousand years. But Lyell thought this rate was probably three times too large; so