The rocks tell their own story, partly, but not wholly. They told just enough to keep the early geologists guessing; and only very recently has the guessing come upon the truth.

These things the rocks told:

1. We have come from a distance.

2. We have had our sharp corners worn off.

3. Many of us have deep scratches on our sides.

4. At various places we have been dumped in long ridges, mixed with much earth.

5. A big boulder is often balanced on another one.

The first thing the geologist noted was the fact that these boulders are strangers—that is, they are not the native rocks that outcrop on hillsides and on mountain slopes near where they are found. Far to the north are beds of rock from which this dÉbris undoubtedly came. Could a flood have scattered them as they are found? No, for water sorts the rock dÉbris it deposits, and it rounds and polishes rock fragments, instead of scratching and grooving them and leaving them angular, as these are.

Professor Agassiz went to Switzerland and studied the glaciers. He found unsorted rock fragments where the glacier's nose melted, and let them fall. They were worn and scratched and grooved, by being frozen into the ice, and dragged over the rocky bed of the stream. The rocky walls of the valley were scored by the glacier's tools. Rounded domes of rock jutted out of the ground, in the paths of the ice streams, just like the granite outcrop in Central Park in New York, and many others in the region of scattered boulders.

After long studies in Europe and in North America, Professor Agassiz declared his belief that a great ice-sheet once covered the northern half of both countries, rounding the hills, scooping out the valleys and lake basins, and scattering the boulders, gravel, and clay, as it gradually melted away.

The belief of Professor Agassiz was not accepted at once, but further studies prove that he guessed the riddle of the boulders. The rich soil of the Northern States is the glacial drift—the mixture of rock fragments of all sizes with fine boulder clay, left by the gradual melting of the great ice-sheet as it retreated northward at the end of the "Glacial Epoch."