[Footnote 19: Did you suspect the giants of this chapter were our old friends the glaciers of the Ice Age, when I first began talking about them?] When the geologists first began digging into these hills they not only found them as full of pebbles as a Christmas pudding is full of plums, but the pebbles were of all kinds--sandstone, limestone, slate, granite. JACK FROST DIDN'T DO IT! "These different pieces of stone didn't come from the breaking up by frost of the rock beds on which we now find them," said Some Wise Man, "for then they would all have been of the same kind of rock." "And besides," said Some Wise Man No. 2, "they would not have been shaped into pebbles with the edges rounded off, as all pebbles are by the waves of lakes or the sea or the water of flowing streams. So these pebbles must have come from somewhere else." "Yes, and a long way off," remarked Some Wise Man No. 3; "for look, there aren't any rock beds anywhere around here from which some of these pebbles could have been made." "True enough," said Wise Man No. 4, "and I know what brought these little foreigners. It was a great flood; for water moves not only pebbles and clay, but, in times of flood, good-sized cobblestones." WHAT IS MEANT BY THE "DRIFT" THEORY So, for a long time, it was believed that the material in these hills was drifted in by the waters. This was called the "drift" theory, and, although it is now known that this theory was not the true one, such heaps of clay and stones are still called "drift." But the learned men kept on digging into the question and into the hills, and finally more things were observed. "Did you notice this?" said one. "The material is not separated into layers and divided up into coarse, finer, finest as the sediment of pebbles, sand, and mud is separated and divided when it settles along shores. These pebbles, this sand and clay, are all mixed up." "Look at this, will you?" (Here imagine a Learned Somebody picking up a pebble with a scratched face like mine.) "Water never scratched anything like that. Here are a lot more of these pebbles, all with their faces scratched." "And just see how all these scratched pebbles have flat faces," cried another of these famous grown-up boys in these great field excursions. "It looks to me as if they had been ground against something hard--another rock, say; and for a long time." HOW THE QUESTION WAS FINALLY SETTLED Well, to make a long story short, they found that the glaciers of the Ice Age, those great bodies of flowing ice, were the only things that could have brought all this material together from such widely separated regions (as shown by the different kinds of pebbles), and left them all mixed up as they were; and the faces of many pebbles scratched and flattened where they had been ground along.