of the delta of the
Mississippi which now is a part of the States of Louisiana and
Mississippi was carried down during the ice-melting period. Dr.
Wright--as we have before stated--has estimated that there are a million
square miles of country that has been covered to an average depth of
fifty feet with glacial drift. A very large amount of the earth that was
spread over the northern portion of the United States by leveling down
hills and mountains in the northern country and scooping out the great
lakes has been carried much farther than to the margin of the ice sheet.
And I have no doubt but that a great portion of Louisiana and western
Mississippi is made of earth carried down largely during the period of
melting ice and deposited in this great delta.

Imagine the effect that would be produced by the giving way of an ice
dam or a great number of them at different periods, that would allow a
body of water as large or larger than Lake Michigan to be drained off in
a comparatively short time. When we think of it in this light the great
delta of the Mississippi is easily accounted for.

There are evidences of a great lake in the Red River country of the
Northwest that is much larger than any of our greatest lakes. The
shores of this lake--the bed of which is now dry land and the heart of a
great agricultural region--are well defined and have been surveyed and
mapped out. When this great body of water was released it was to the
northward. For this reason it was undoubtedly held for a much longer
time than some of the lakes to the southward where the ice melted
sooner.

CHAPTER XXVIII.

SOME EFFECTS OF THE GLACIAL PERIOD.

There is a wonderfully interesting effect produced by the action of
water during the subsidence of a glacier at Lucerne, Switzerland. Some
years ago there was discovered under a pile of glacial drift at the edge
of the town of Lucerne a number of deep holes worn in a great ledge of
rocks that crop out at that point. One of these pot-holes having been
discovered, excavations were continued until a large number of them were
unearthed of various shapes and sizes. I had the pleasure of inspecting
some of them in the year 1881. They are situated within an inclosure
called the Garden of the Glaciers. Some of these holes are twenty to
thirty feet in diameter, and the same depth. There are others that are
smaller in size, but all of them possess the same general
characteristics.

In the bottom o