Part 7 (1/2)

The next step in our history of glaciation is to outline the disappearance of the ice sheets. When a decrease in solar activity produced a corresponding decrease in storminess, several influences presumably combined to cause the disappearance of the ice. Most of their results are the reverse of those which brought on glaciation. A few special aspects, however, some of which have been discussed in _Earth and Sun_, ought to be brought to mind. A diminution in storminess lessens upward convection, wind velocity, and evaporation, and these changes, if they occurred, must have united to raise the temperature of the lower air by reducing the escape of heat. Again a decrease in the number and intensity of tropical cyclones presumably lessened the amount of moisture carried into mid-lat.i.tudes, and thus diminished the precipitation. The diminution of snowfall on the ice sheets when storminess diminished was probably highly important. The amount of precipitation on the sheets was presumably lessened still further by changes in the storminess of middle lat.i.tudes. When storminess diminishes, the lows follow a less definite path, as Kullmer's maps show, and on the average a more southerly path. Thus, instead of all the lows contributing snow to the ice sheet, a large fraction of the relatively few remaining lows would bring rain to areas south of the ice sheet. As storminess decreased, the trades and westerlies probably became steadier, and thus carried to high lat.i.tudes more warm water than when often interrupted by storms. Steadier southwesterly winds must have produced a greater movement of atmospheric as well as oceanic heat to high lat.i.tudes. The warming due to these two causes was probably the chief reason for the disappearance of the European ice sheet and of those on the Pacific coast of North America. The two greater American ice sheets, however, and the glaciers elsewhere in the lee of high mountain ranges, probably disappeared chiefly because of lessened precipitation. If there were no cyclonic storms to draw moisture northward from the Gulf of Mexico, most of North America east of the Rocky Mountain barrier would be arid. Therefore a diminution of storminess would be particularly effective in causing the disappearance of ice sheets in these regions.

That evaporation was an especially important factor in causing the ice from the Keewatin center to disappear, is suggested by the relatively small amount of water-sorted material in its drift. In South Dakota, for example, less than 10 per cent of the drift is stratified.[45] On the other hand, Salisbury estimates that perhaps a third of the Labradorean drift in eastern Wisconsin is crudely stratified, about half of that in New Jersey, and more than half of the drift in western Europe.

When the sun's activity began to diminish, all these conditions, as well as several others, would cooperate to cause the ice sheets to disappear.

Step by step with their disappearance, the amelioration of the climate would progress so long as the period of solar inactivity continued and storms were rare. If the inactivity continued long enough, it would result in a fairly mild climate in high lat.i.tudes, though so long as the continents were emergent this mildness would not be of the extreme type.

The inauguration of another cycle of increased disturbance of the sun, with a marked increase in storminess, would inaugurate another glacial epoch. Thus a succession of glacial and inter-glacial epochs might continue so long as the sun was repeatedly disturbed.

FOOTNOTES:

[Footnote 38: This chapter is an amplification and revision of the sketch of the glacial period contained in The Solar Hypothesis of Climatic Changes; Bull. Geol. Soc. Am., Vol. 25, 1914.]

[Footnote 39: R. D. Salisbury: Physical Geography of the Pleistocene, in Outlines of Geologic History, by Willis, Salisbury, and others, 1910, p.

265.]

[Footnote 40: The Quaternary Ice Age, 1914, p. 364.]

[Footnote B: For fuller discussion of climatic controls see S. S.

Visher: Seventy Laws of Climate, Annals a.s.soc. Am. Geographers, 1922.]

[Footnote 41: Many of these alterations are implied or discussed in the following papers:

1. F. W. Harmer: Influence of Winds upon the Climate of the Pleistocene; Quart. Jour. Geol. Soc., Vol. 57, 1901, p. 405.

2. C. E. P. Brooks: Meteorological Conditions of an Ice Sheet; Quart.

Jour. Royal Meteorol. Soc., Vol. 40, 1914, pp. 53-70, and The Evolution of Climate in Northwest Europe; _op. cit._, Vol. 47, 1921, pp. 173-194.

3. W. H. Hobbs: The Role of the Glacial Anticyclone in the Air Circulation of the Globe; Proc. Am. Phil. Soc., Vol. 54, 1915, pp.

185-225.]

[Footnote 42: W. B. Wright: The Quaternary Ice Age, 1914, p. 100.]

[Footnote 43: The description of the distribution of the ice sheet is based on T. C. Chamberlin's wall map of North America at the maximum of glaciation, 1913.]

[Footnote 44: Chamberlin and Salisbury: Geology, 1906, Vol. 3, and W. H.

Hobbs: Characteristics of Existing Glaciers, 1911.]

[Footnote 45: S. S. Visher: The Geography of South Dakota; S. D. Geol.

Surv., 1918.]

CHAPTER VIII

SOME PROBLEMS OF GLACIAL PERIODS

Having outlined in general terms the coming of the ice sheets and their disappearance, we are now ready to discuss certain problems of compelling climatic interest. The discussion will be grouped under five heads: (I) the localization of glaciation; (II) the sudden coming of glaciation; (III) peculiar variations in the height of the snow line and of glaciation; (IV) lakes and other evidences of humidity in unglaciated regions during the glacial epochs; (V) glaciation at sea level and in low lat.i.tudes in the Permian and Proterozoic eras. The discussion of perhaps the most difficult of all climatic problems of glaciation, that of the succession of cold glacial and mild inter-glacial epochs, has been postponed to the next to the final chapter of this book. It cannot be properly considered until we take up the history of solar disturbances.

I. The first problem, the localization of the ice sheets, arises from the fact that in both the Pleistocene and the Permian periods glaciation was remarkably limited. In neither period were all parts of high lat.i.tudes glaciated; yet in both cases glaciation occurred in large regions in lower lat.i.tudes. Many explanations of this localization have been offered, but most are entirely inadequate. Even hypotheses with something of proven worth, such as those of variations in volcanic dust and in atmospheric carbon dioxide, fail to account for localization. The cyclonic form of the solar hypothesis, however, seems to afford a satisfactory explanation.

The distribution of the ice in the last glacial period is well known, and is shown in Fig. 6. Four-fifths of the ice-covered area, which was eight million square miles, more or less, was near the borders of the North Atlantic in eastern North America and northwestern Europe. The ice spread out from two great centers in North America, the Labradorean east of Hudson Bay, and the Keewatin west of the bay. There were also many glaciers in the western mountains, especially in Canada, while subordinate centers occurred in Newfoundland, the Adirondacks, and the White Mountains. The main ice sheet at its maximum extension reached as far south as lat.i.tude 39 in Kansas and Kentucky, and 37 in Illinois.