Next day they came to a canyon that filled the Geological Survey man with profound enthusiasm, for, he said, it illustrated both the last glacial period and the last period of volcanic mountain building. First they noted that the little mountain stream had worn its torrential way through the basalt or volcanic rock in a narrow canyon perhaps 200 feet deep. A flow of molten basalt, accompanied by cinders, had been erupted from the 8,000 foot peak at the upper end of the canyon, and had flowed down in a layer 200 feet thick when it hardened. It had flowed,—as the underlying rock still showed in places,—over a lateral moraine or rock débris left by a glacier as it flowed down that way. And from the weathered condition of this rock débris, Norris said, it must have been a glacier, not of the last ice age, but of the one preceding,—for of the four glacier periods generally recognized by geologists to-day, evidences of the last two can be seen in the Sierras.
What made this little canyon even more of a find, (from the point of view of what he wanted to show the boys), was that on top of the volcanic rock lay the deposit from another glacier, one that flowed in the last ice age, as the condition of the rock débris plainly showed the expert.
The boys tucked a few rock specimens into their packs and launched an avalanche of questions. But he made them wait till they had established all snug for the night beside a stretch of rapids, where they could look forward to catching trout for breakfast. Then, lighting his pipe, and stretching his feet to the bon-fire,—for the night wind swept cool upon them,—Norris began with Ted’s question as to glaciers and volcanoes.
“During the times I spoke of last night, when the earth crust is breaking, the molten rock and gases and water vapor in the interior of the planet rise in the hearts of the mountain ranges, and often break through as active volcanoes, pouring their lava and ash over the underlying granite, and building it still higher.
“These heightened mountain ranges bring about the glacial climates. For the snows on their cold peaks do not melt when summer comes, and consequently they accumulate, and accumulate, till their own weight presses them down as hard as ice,—that is, makes glaciers of them. I am going to be on the look-out for a glacier, for you will have a good chance to see them in this region. At the same time, during these glacial periods, the astronomer could explain how it is that the temperature is from ten to twenty degrees colder in both winter and summer than it is now, so that helps the ice to accumulate. Then the glacier, flowing slowly, slowly, (a river of ice), down the mountainsides, carries with it quantities of the underlying rocks, till it reaches a lower level where the ice melts and it becomes a river and carries those rocks and soil to the sea. That way, the mountains are gradually worn down to sea level and the whole cycle is ready to start over again.”
“I see,” said the ranch boy. “How long ago did you say the last glacier period came?”
“Probably not since the time of the first men,—perhaps 30,000 years ago.”
“And those glacial deposits you showed us to-day are 30,000 years old?” the boy breathed.
“Yes, and the deposits from the glacial period before that are older still,—a souvenir from the age of reptile dominance.”
“Then when did the other ice ages come? Did you say there were five?”