Man and the Glacial Period

by G. Frederick Wright

It will be best to turn again to the map to observe the possible effect upon the Gulf Stream of a geological event of which we have some definite evidence, and which is adduced by Mr. Upham and others as one of the important probable causes of the Glacial period, namely, the subsidence of the Isthmus of Panama and the adjacent narrow neck of land connecting North with South America. It will be seen at a glance that a subsidence sufficient to allow the northwest current of warm water, pushed by the trade-winds along the northeast shore of South America, to pass into the Pacific Ocean, instead of into the Gulf of Mexico, would be a cause sufficient to produce the most far-reaching results; it would rob the North Atlantic of the immense amount of heat and moisture now distributed over it by the Gulf Stream, and would add an equal amount to the northern Pacific Ocean, and modify to an unknown extent the distribution of heat and moisture over the lands of the northern hemisphere.

The supposition that a subsidence of the Isthmus of Panama was among the contributing causes of the Glacial period has been often made, but without any positive proof of such subsidence. From evidence which has recently come to light, however, it is certain that there has actually been considerable subsidence there in late Tertiary if not in post-Tertiary times. This evidence is furnished by Dr. G. A. Maack and Mr. William M. Gabb in their report to the United States Government in 1874 upon the explorations for a ship-canal across the isthmus, and consists of numerous fossils belonging to existing species which are found at an elevation of 150 feet above tide. As the dividing ridge is more than 700 feet above tide, this does not positively prove the point, but so much demonstrated subsidence makes it easy to believe, in the absence of contradictory evidence, that there was more, and that the isthmus was sufficiently submerged to permit a considerable portion of the warm equatorial current which now passes northward from the Caribbean Sea and the Gulf of Mexico to pass into the Pacific Ocean.

Fig. 101.—Map showing how the land clusters about the north pole.

An obvious objection to the theory of a late Tertiary or post-Tertiary subsidence of the Isthmus of Panama presents itself in the fact that there is at present a complete diversity of species between the fish inhabiting the waters upon the different sides of the isthmus. If there had been such a subsidence, it seems natural to suppose that Atlantic species would have migrated to the Pacific side and obtained a permanent lodgment there, and that Pacific species would have found a congenial home on the Atlantic side. It must be confessed that this is a serious theoretical difficulty, but perhaps not insuperable. For it is by no means certain that colonists from the heated waters of the Caribbean Sea would become so permanently established upon the Pacific side that they could maintain themselves there upon the re-establishment of former conditions. On the contrary, it seems reasonable to suppose that upon the re-elevation of the isthmus the northern currents, which would then resume their course, would bring back with them conditions unfavourable to the Atlantic species, and favourable to the competing species which had only temporarily withdrawn from the field, and which might now be better fitted than ever to renew the struggle with their Atlantic competitors. It is by no means certain, therefore, that with the re-establishment of the former conditions there would not also be a re-establishment of the former equation of life upon the two sides of the isthmus.

Mr. Upham’s theory involves also extensive elevations of land in the northern part of America; in this respect agreeing with the opinions early expressed by Professors J. D. Dana and J. S. Newberry. Of the positive indications of such northward elevations of land we have already spoken when treating in a previous chapter of the fiords and submerged channels which characterise northern Europe and both the eastern and the western coasts of North America. But in working out the problem the solution is only half reached when we have got the Gulf Stream into the Pacific Ocean, and the land in the northern part of the continents elevated to some distance above its present level. There is still the difficulty of getting the moisture-laden currents from the Pacific Ocean to carry their burdens over the crest of the Sierra Nevada and Rocky Mountains and to deposit them in snow upon the Laurentian highlands. An ingenious supplement to the theory, therefore, has been brought forward by Professor Carpenter, who suggests that the immense Tertiary and post-Tertiary lava-flows which cover so much of the area west of the Rocky Mountains were the cause of the accumulations of snow which formed the Laurentide Glacier. This statement, which at first seems so paradoxical as to be absurd, appears less so upon close examination.

The extent of the outflows of lava west of the Rocky Mountains is almost beyond comprehension. Literally, hundreds of thousands of square miles have been covered by them to a depth in many places of thousands of feet. These volcanic eruptions are mostly of late date, beginning in the middle of the Tertiary and culminating probably about the time of the maximum extent of the Laurentide Glacier. Indeed, so nearly contemporaneous was the growth of the Laurentide Glacier with these outflows that Professor Alexander Winchell had, with a good deal of plausibility, suggested that the outflows of the eruptions of lava were caused by the accumulation of ice over eastern British America. His theory was that the three million cubic miles of ice which is proved to have been abstracted from the ocean and piled up over that area was so serious a disturbance of the equilibrium of the earth’s crust that it caused great fissures to be opened along the lines of weakness west of the Rocky Mountains, and pressed the liquid lava out, as the juice is pressed out of an orange in one place by pressing upon the rind in another.

Professor Carpenter’s view is the exact reverse of Professor Winchell’s. Going back to those orographic changes which produced the lava-flows and the elevation of the northern part of British America, he thinks the problem of getting the moisture transferred from the Pacific Ocean to the Canadian highlands is solved by the lava-flows west of the Rocky Mountains. This immense exudation of molten matter was accompanied by an enormous liberation of heat, which must have produced significant changes in the meteorological conditions.