Man and the Glacial Period

by G. Frederick Wright

The moisture of the atmosphere is precipitated by means of the condensation connected with a lowering of its temperature. Ordinarily, therefore, when moist winds from an oceanic area pass directly over a lofty mountain-chain, the precipitation takes place immediately, and the water finds its way back by a short course to the sea. This is what now actually occurs on the Pacific coast. The Sierra Nevada condense nearly all the moisture; so that very little falls on the vast area extending from their summits eastward to the Rocky Mountains. All that region is now practically a desert land, where the evaporation exceeds the precipitation. In Professor Carpenter’s view the heat radiated from the freshly exuded lava is supposed to have prevented the precipitation near the coast-line, and to have helped the winds in carrying it farther onward to the northeast, where it would be condensed upon the elevated highlands, upon which the snows of the great Laurentide Glacier were collected.

It is not necessary for us to attempt to measure the amount of truth in this subsidiary hypothesis of Professor Carpenter, but it illustrates how complicated are the conditions which have to be considered before we rest securely upon any particular hypothesis. The unknown elements of the problem are so numerous, and so far-reaching in their possible scope, that a cautious attitude of agnosticism, with respect to the cause of the Glacial period, is most scientific and becoming. Still, we are ready to go so far as to say that Mr. Upham’s theory comes nearest to giving a satisfactory account of all the phenomena, and it is to this that Professor Joseph Le Conte gives his cautious approval.

Summarily stated, this theory is, that the passage from the Tertiary to the Quaternary or Glacial period was characterised by remarkable oscillations of land-level, and by corresponding changes of climate, and of ice-accumulation in northern regions; that the northern elevation was connected with subsidence in the equatorial regions; that these changes of land-level were both initiated and, in the main, continued by the interior geological forces of the globe; but that the very continental elevation which mainly brought on the Glacial period added at length, in the weight of the ice which accumulated over the elevated region, a new force to hasten and increase the subsidence, which would have taken place in due time in the natural progress of the orographic oscillations already begun. Professor Le Conte illustrates the subject by the following diagram, which, for simplicity’s sake, treats the Glacial epoch as one; the horizontal line, A B, represents time from the later Pliocene until now; but it also represents the present condition of things both as to land-level and as to ice-accumulation. The full line, c d e, represents the oscillations of land (and presumably of temperature) above and below the present condition. The broken line represents the rise, culmination, and decline of ice-accumulation. The dotted line represents the crust-movement as it would have been if there had been no ice-accumulation.

Fig. 102.

Succession of Epochs, Glacial and Fluvial Deposits, and
Changes in Altitude and Climate, during the Quaternary Era.

Epochs.	Eastern Provinces and New England.	Middle and Southern Atlantic States.
Recent or Terrace. (Mostly within the period of traditional and written history.)	Rise of the land to its present height, or somewhat higher, soon after the departure of the ice. Rivers eroding their glacial flood-plains, leaving remnants as terraces. Warmer climate than now, probably due to greater Gulf Stream, formerly permitted southern mollusks to extend to Gulf of St. Lawrence, now represented by isolated colonies.	Continued subsidence of coast at New York and southward, and rise of the mountainous belt, by displacement along the fall line of the rivers. Much erosion of the Columbia formation since culmination of second Glacial epoch; sedimentation in bays, sounds, and estuaries.
Glacial Period or Ice Age. Pleistocene Period.
Champlain. (Close of the second Glacial epoch.)	Land depressed under ice-weight; glacial recession; continued deposition of upper till and deep flood-plains of gravel, sand and clay (modified drift). Terminal moraines marking pauses or readvance during general retreat of ice. Marine submergence. 150 to 230 feet on coast of Maine, to 520 feet in Gulf and valley of St. Lawrence.	Less subsidence in latitude of New York and southward than at north; lower Hudson Valley, and part of its present submarine continuation, above sea-level. Gravel and sand deposits from englacial drift in Delaware and Susquehanna Valleys, inclosing abundant human implements at Trenton, N.J.
Second Glacial.	Second great uplift of the land, 3,000 to 4,000 feet higher than now; snow-fall again all the year; ice probably two miles thick on Laurentide highlands, and extending somewhat farther south here than in first glaciation. Lower till (ground moraine), and upper till (englacial drift). Terminal moraines, kames, osars, valley drift.	Renewal of great continental elevation (3,000 feet in latitude of New York and Philadelphia), of excessive snow-fall and rains, and of wide-spread fluvial deposits, the Columbia formation, on the coastal plain, during early part of this epoch. Implements of man at Claymont, Del.
Inter-glacial. (Longest epoch of this era.)	Ice-sheet melted here; probably not more ice in arctic regions than now. Fluvial and lacustrine deposits of this time, with those of the first Glacial epoch, were eroded by the second glaciation.	Depression, but generally not to the present level. Deep channels cut in the bed-rocks by the Delaware, Susquehanna, Potomac, and other rivers. The Appomattox deposits much eroded. Relative length of this epoch made known by McGee from study of this region.
First Glacial.	Begun by high continental uplift, cool climate and snow-fall throughout the year, producing ice-sheet. Much glacial erosion and transportation; till and stratified deposits. Ended by depression of land; return of warm climate, with rain; final melting of the ice. Isthmus of Panama probably submerged (Gulf Stream smaller), and again in second Glacial epoch.	Continental elevation; erosion of Delaware and Chesapeake Bays, and of Albemarle and Pamlico Sounds. Plentiful snow-fall on the southern Appalachian Mountains; snows melted in summer, and heavy rains, producing broad river-floods, with deposition of the Appomattox formation.
Succession of Epochs (cont.)
Mississippi Basin and northward.	Cordilleran Region.	Europe and Asia.
Terracing of river valleys. Northward rise of area of Lake Agassiz nearly complete before the ice was melted on the country crossed by Nelson River; but rise about Hudson Bay is still going on; 7,000 to 8,000 years since ice-melting uncovered Niagara and falls of St. Anthony.	Including a stage of considerable uplift, with return of humid conditions, Alpine glaciation (third Glacial epoch), and the second great rise of Lakes Bonneville and Lahontan. Very recent subsidence and change to present aridity.	Erosion and terracing of stratified drift in river valleys. Land passage of European flora to Greenland; succeeded by subsidence there, admitting warm currents to Arctic Sea. Minor climatic changes, including a warmer stage than now. Upper and outer portions of Indo-Gangetic alluvial plain; extensive deposits of Hwang Ho, and destructive changes of its course.
Glacial Period or Ice Age. Pleistocene Period.
Abundant deposition of englacial drift. Stone implements in river gravels of Ohio, Ind., and Minn. Laurentian lakes held at higher levels, and Lake Agassiz formed in Red River basin, by barrier of retreating ice, with outlets over lowest points of their present southern water-shed. Marine submergence 300 to 500 feet on southwest side of Hudson Bay.	Depression probably almost to the present level. Restoration of arid climate; nearly or quite complete evaporation of Lakes Bonneville and Lahontan. Formation of the “adobe” continuing through the second Glacial, Champlain, and Recent epochs.	Final departure of the ice-sheets; glacial rivers forming eskers and kames. Loess deposited while the region of the Alps was depressed lower than now. Upper (englacial) till, and asar, of Sweden. Marine submergence 500 to 600 feet in Scotland, Scandinavia, and Spitzbergen.
Ice-sheet here less extensive than in the first Glacial epoch, and not generally bordered as then by lakes in valleys which now drain southward. Terminal moraines at extreme limit of the ice-advance, and at ten or more stages of halt or readvance in its retreat.	Probable uplift 3,000 feet, shown by submerged valleys near Cape Mendocino. Second ice-sheet on British Columbia and Vancouver Island; local glaciation of Rocky Mountains, Cascade range, and Sierra Nevada, south to latitude 37°. First great rise of Lakes Bonneville and Lahontan.	Second elevation and general glaciation of northwestern Europe; the ice-sheets of Great Britain probably more extensive than in first Glacial epoch. Oscillations of ice-front; British Lower and Upper bowlder-clays, the Chalky, Purple, and Hessle bowlder-clays. Terminal moraines in Germany.
Depression nearly to present level southward; more northward, but followed there, by differential uplift of 800 or 1,000 feet. Great erosion of loess and other modified drift, and of “Orange Sand.” Valleys of this epoch, partly filled with later till, are marked by chains of lakes in southern Minnesota.	Continental depression. Arid climate. Long-continued denudation of the mountains: resulting very thick subaërial deposits of the “adobe.” Intermittent volcanic action in various parts of this region, throughout the Quaternary era to very recent times, and liable to break forth again.	Recession, or probably complete departure, of the ice-sheets. Land connection between Europe and Africa, permitting southern animals to extend far northward. Erosion of the Somme Valley below its oldest implement-bearing gravels.
Pliocene elevation of continent brought to culmination at beginning of Quaternary era; this whole basin probably then uplifted 3,000 feet; excessive snow-fall and rain; deposition of the “Orange Sand.” Ice-sheet south to Cincinnati and St. Louis, at length depressing the earth’s crust beneath it; slackened river floods and shallow lakes, forming the loess.	Latest rise (3,000 feet) of the Colorado Cañon district. Sierra Nevada and other Great Basin mountain-ranges formed by immense uplifts, with faulting. California river-courses changed; human bones and implements in the old river gravels, lava-covered. Ice-sheet on British Columbia; local glaciers southward.	Uplift and glaciation of northwestern Europe: maximum elevation. 2,500 feet or more (depth of the Skager Rack); France and Britain united with the Färöe Islands, Iceland, and Greenland. Uplifts of the Himalayas and other mountain-ranges attendant on both Glacial epochs.