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The process of raising the mountains is supposed to have been very slow and gradual. On the other hand, it is clear that igneous rock, already hard, had to become fluid in order to penetrate sedimentary rock or cover it. It is not known what initiated this process, but it is asserted that it must have happened long before man appeared on the earth. So when skulls of early man are found in late deposits, or skulls of modern man are found together with bones of extinct animals in early deposits, difficult problems are presented. Occasionally, also, during mining operations, a human skull is found in the middle of a mountain, under a thick cover of basalt or granite, like the Calaveras skull of California.
Human remains and human artifacts of bone, polished stone, or pottery are found under great deposits of till and gravel, sometimes under as much as a hundred feet.
The origin of clay, sand, and gravel on igneous and sedimentary rock, offers a problem. The theory of Ice Ages was put forth (1840) to explain this and other enigmatic phenomena. As far north as Spitzbergen, in the polar circle, at some time in the past, coral reefs were formed, which do not occur except in tropical regions; palms also grew on Spitzbergen. The continent of Antarctica, which today has not a single tree on it, must have been covered at one time by forests, since it has coal deposits.
As we see, the planet earth is full of secrets. We have not come closer to solving the problem of the origin of the solar system by investigating the planet under our feet; on the contrary, we have found many other unsolved problems concerning the lithosphere, hydrosphere, and atmosphere of the earth. Shall we be more fortunate if we try to understand the process that caused the changes on the globe in the most recent geological epoch, the time of the last glacial period, a period close to the time which is regarded as historical?
Ice Ages
Not many thousands of years ago, we are taught, great areas of Europe and of North America were covered with glaciers. Perpetual
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on the globe we are faced with it. Here is how the question is put concerning the eastern coast of North America.
"Not long ago in a geological sense, the flat plain from New Jersey to Florida was under the sea.
At that time the ocean surf broke directly on the Old Appalachian Mountains. Previously the southeastern part of the mountain structure had sunk below the sea and become covered with a layer of sand and mud, thickening seaward. The wedgelike mass of marine sediments was then uplifted and cut into by rivers, giving the Atlantic coastal plain of the United States. Why was it uplifted? To the westward are the Appalachians. The geologist tells us of the stressful times when a belt of rocks extending from Alabama to Newfoundland was jammed, thrust together, to make this mountain system. Why? How was it done? In former times the sea flooded the region of the great plains from Mexico to Alaska, and then withdrew. Why this change?" 3
The birth of the Cordilleras—"again the mystery of mountain-making clamors for solution."
And so on all over the world. The Himalayas were under the sea. Now Eurasia is three miles or more above the bottom of the Pacific. Why?
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"The problem of mountain-making is a vexing one: many of them [mountains] are composed of tangentially compressed and over-thrust rocks that indicate scores of miles of circumferential shortening in the Earth's crust. Radial shrinkage is woefully inadequate to cause the observed amount of horizontal compression. Therein lies the real perplexity of the problem of mountain-making. Geologists have not yet found a satisfactory escape from this dilemma." *
Even authors of textbooks confess their ignorance. "Why have sea floors of remote periods become the lofty highlands of today? What generates the enormous forces that bend, break, and mash the rocks in mountain zones? These questions still await satisfactory answers."5
3 R. A. Daly; Our Mobile Earth (1926), p. 90.
4 F. K. Mather, Review of Biography of the Earth by G. Gamovr, Science, Jan. 16, 1942.
5 C. R. Longwell, A. Knopf, and R. F. Flint, A Textbook of Geology (1939), p. 405.
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ice lay not only on the slopes of high mountains, but loaded itself in heavy masses upon continents even in moderate latitudes. Where today the Hudson, the Elbe, and the upper Dnieper flow, there were then frozen deserts. They were like the immense glacier of Greenland that covers that island. There are signs that a retreat of the glaciers was interrupted by a new massing of ice, and that their borders differed at various times. Geologists are able to find the boundaries of the glaciers. Ice moves very slowly, pushing stones before it, and accumulations of stones or moraines remain when the glacier retreats melting away.
Traces have been found of five or six consecutive displacements of the ice sheet during the Ice Age, or of five or six glacial periods. Some force repeatedly pushed the ice sheet toward moderate latitudes. Neither the cause of the ice ages nor the cause of the retreat of the icy desert is known; the time of these retreats is also a matter of speculation.
Many ideas were offered and guesses made to explain how the glacial epochs originated and why they terminated. Some supposed that the sun at different times emits more or less heat, which causes periods of heat and cold on the earth; but no evidence that the sun is such a "variable star"
was adduced to support this hypothesis.
Others conjectured that cosmic space has warmer and cooler areas, and that when our solar system travels through the cooler areas, ice descends upon latitudes closer to the tropics. But no physical agents were found responsible for such hypothetical cold and warm areas in space.
A few wondered whether the precession of the equinoxes or the slow change in the direction of the terrestrial axis might cause periodic variations in the climate. But it was shown that the difference in insolation could not have been great enough to have been responsible for the glacial ages.
Still others thought to find the answer in the periodic variations in the eccentricity of the ecliptic (terrestrial orbit), with glaciation at the maximal eccentricity. Some of them supposed that winter in aphelion, the remotest part of the ecliptic, would cause glaciation; and some thought that summer in aphelion would produce that effect.
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Some scholars thought about the changes in the position of the terrestrial axis. If the planet earth is rigid, as it is regarded to be (L. Kelvin), the axis could not have shifted in geological times by more than three degrees (George Darwin); if it were elastic, it could have shifted up to ten or fifteen degrees in a very slow process.
The cause of the ice ages was seen by a few scholars in the decrease of the original heat of the planet; the warm periods between the ice ages were attributed to the heat set free by a hypothetical decomposition of organisms in the strata close to the surface of the ground. The increase and decrease in the action of warm springs were also considered.
Others supposed that dust of volcanic origin filled the terrestrial atmosphere and hindered insolation, or, contrariwise, that an increased content of carbon dioxide in the atmosphere obstructed the reflection of heat rays from the surface of the planet. A decrease in the amount of carbon dioxide in the atmosphere would cause a fall of temperature (Arrhenius), but calculations were made to show that this could not be the real cause of the glacial ages (Angstrom).
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Changes in the direction of warm currents in the Atlantic Ocean were brought into the discussion, and the Isthmus of Panama was theoretically removed to allow the Gulf Stream to pass into the Pacific at the time of the glacial periods. But it was proved that the two oceans were already divided in the Ice Age; besides, a part of the Gulf Stream would have remained in the Atlantic anyway. The periodic retreats of ice between the glacial periods would have required periodic removal and replacement of the Isthmus of Panama.