It was in much the same way that Mr. Labrador and his friend Keewatin did that great piece of engineering at the Great Lakes. Where the Great Lakes are now there used to be rivers that were a part of the St. Lawrence system. Then along came the ice sheets, dammed up these rivers, just as small boys dam up roadside rivulets after a rain, and so made big lakes, as the boys make little lakes in these streamlets. But this wasn't all. The glaciers evidently wanted these to be nice big lakes that would stay there for people to ride on in the beautiful summer weather, and to help haul coal and iron ore and other kinds of freight--Michigan peaches and everything. For look what else they did. With pebbles and big stones and dirt they built the lake walls higher, and dug deep basins for them out of the solid rock. Then they poured in a lot of extra water--beautiful blue water, tons and tons of it--and went back home. The digging into the rock was done with big chisels--what a carpenter would call "round-nosed" chisels. These chisels, of course, were made of ice. They were what are called the "tongues" or "lobes" of glaciers. As a glacier flows along--always on some down grade--there are portions of it--those long lobes or tongues--that move on ahead of the main mass. This is because those parts of the ice sheet strike a steeper bit of land than the rest of it, so how could they help moving faster? [Illustration: THE THOUSAND-YEAR CLOCK AT NIAGARA You've heard of eight-day clocks and clocks that have to be wound only once a year, but here is a clock that was wound up several thousand years ago and is still going beautifully! In placing the wondrous waterfall in Niagara River the glaciers also started a kind of water-clock by which to record--for those who would take the trouble to study it out--how long ago it was the glaciers visited us. Owing to the constant wearing away of the base of the falls, by the water grinding the pebbles against it, great blocks like the one here shown (known as "The Rock of Ages") come tumbling down. So the falls are constantly retreating up-stream, and the distance from where they once stood to where they are now gives a rough idea of the time that has passed since the Old Men of the Mountain set them up in business--about 25,000 years. ] The fronts of these lobes are rounded like the waves flowing up a beach, or syrup travelling over pancakes on a cold winter morning. The reason of this roundness is that the centres of these lobes of ice or water travel fastest because the mass on either side furnishes a kind of ball-bearing for the central part.