Page:Popular Science Monthly Volume 49.djvu/178

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164
THE POPULAR SCIENCE MONTHLY.

ated by the tilting of the land, which will be noted later. The closing of the old water ways ends the history of the ancient Laurentian River. When the river began to flow again, the lacustrine epoch was established.

Submergence and Re-elevation of the Lake District.— After the obstruction of the valleys with drift, the whole lake

PSM V49 D178 Cut terrace and beach section.jpg
Fig. 6.—Section showing the Floor of a Cut Terrace on which rests a Beach. b and c beaches broken into ridgelets; d, a frontal sand bar; W, old water level.

region was submerged; but this depression is best treated of in the rising of the land which has brought the evidence to view.

As the waves beat upon the shores, the beaches, terraces, sea cliffs, sea caves, etc, become characteristic of coast lines. When the water sinks or the land rises, the various stages of the deserted shore remain and record the recession of the waves. The preservation of the old coast lines is often so perfect as to furnish easy identification of their character, as may be seen in Figs. 6 and 7, which represent sections of old beaches. Behind them lagoons often occur, and the entrances of bays are often barred across with beaches, as is shown in Fig. 8.

The resemblance between modern and ancient shores is further illustrated in Figs. 9 and 10, where great bowlder pavements are

PSM V49 D178 Cut terrace section without beach.jpg
Fig, 7.—Section showing the Floor of a Cut Terrace without Beach but with Bowlder Pavement. P, bowlder pavement; W, old water level.

shown, marking the modern and deserted strands. In valleys, although broad, beaches do not occur, but they are replaced by terraces.

The deserted beaches and terraces in the lake region occur at all altitudes, where such could be preserved. But in order to find the remains of old shore lines continuous over long distances, it is necessary to descend to the levels where the water was more or less confined in the western and central portions of the lake district, for until a recent date there were no barriers toward the northeast sufficiently high to hold the waters of the lakes above tide. In the prenatal lake epoch, such an embayment called for in the last sentence covered two hundred thousand square miles of the lake region, and has been named Warren Gulf. As there was no land barrier to this gulf in the northeastern direction, and for the