f each one was found a bowlder, and in one or two cases
two of them. The action of the water had given these bowlders a gyratory
motion, which gradually wore away the rock underneath until round holes
were formed to the size and depth heretofore mentioned. Where there was
only a single bowlder the holes were almost perfectly round, but where
there was more than one bowlder the holes were sometimes in an oblong
shape. The bowlders were worn down to a very small size in most cases,
and were round and smooth. The probabilities are that when the action
first began these bowlders were large and of irregular shape. They must
have been, in order to do the enormous amount of grinding that some of
them did to produce excavations in the solid rock with a diameter of
thirty feet and a depth about the same. The bottoms were round like an
old-fashioned pot, and the insides polished perfectly smooth. This was
purely an effect of the tumbling about of the bowlders by the running
water from the melting ice of the great glacier that covered that region
some time in the long ago.

There are other effects produced in rocks during the ice flow in North
America that are very interesting. Great grooves are formed in the
rocks, in many cases running for long distances, that have been worn in
by the cutting power of the great ice sheet during the progress of its
movement. There is a great groove to be seen at Kelly's Island in Lake
Erie. It will be remembered that this lake is supposed to have been
formed entirely by the ice of the glacial period. In its movement
across the country which is now covered by the lake the ice encountered
a huge rock formation at Kelly's Island. Great V-shaped grooves were cut
through this rock by the action of the ice, deep enough for a man to
stand in. In other places the rock was planed off in the form of a great
molding, a number of feet wide, with the same smoothness and accuracy as
though done by a machine.

Another effect of the glacial period has been the creation of numerous
waterfalls throughout the glaciated area. The most notable instance is
that of the Falls of Niagara.

In preglacial times the beds of all rivers and water courses had worn
down to an even slope, so that there were very few, if any, waterfalls
such as we have to-day. As we have before stated, Niagara River as well
as the St. Lawrence River is a new outlet for the drainage of the great
lakes. A part of this drainage formerly had its ou