Cause of the Ice Age

CAUSE OF THE ICE AGE The special instance of the cause of the Ice age is but a special aspect of the general question of climates in geological time—genial conditions as well as frigid—and it is possible that the explanation, whatever it may be, has not yet been formulated though speculation has engaged the minds of astronomers, geolo gists and meteorologists for more than a century. W. B. Wright has discussed with great perspicuity a number of proposals and his work (The Quaternary Ice Age) and C. E. P. Brooks' Evolution of Climate must be consulted by all interested in the question.

The causes suggested by various authors arranged themselves roughly in the two categories of Astronomical and Telluric. Dis regarding the chronological order of their enunciation there is convenience in considering, if only to dismiss, one or two of the first. One cause suggested was the variation of solar radiation. The sun has been considered to be a variable star, and the Sun-spot period gives some countenance to this as a possible cause of variation of the intensity of solar radiation of longer period, but as it is unsupported by definite evidence it cannot be fruitfully discussed. Another explanation is based on the supposition that there are cold regions in space ; this cause, however, is manifestly insufficient ; the amount of radiant heat received from star-shine is entirely inadequate to produce an appreciable change in the climate of the earth. A third is based on changes in the position of the earth's axis of rotation. Sir G. H. Darwin has examined this question and on the hypothesis of an absolutely rigid earth he concluded that the maximum dis placement possible was 3°, but on the assumption of some degree of plasticity it was possible for the pole to wander i o° to 15°, an amount disproportionate to the observed climatic effects.

The Astronomical Theory.

The theory that now holds the field owed its inception to Herschel's Outlines of Astronomy and is based upon the fact that the form of the earth's orbit varies in the degree of its departure from a circle. When the eccen tricity is at a maximum, the effect of the precession of the equi noxes will be to give each hemisphere in turn for a period of 10,50o years a short hot summer and a long cold winter; and this will be followed by another equal period of long mild summers and short winters of moderate intensity. Sir Robert Ball puts the contrasts in this way : the year will be unequally divided into seasons of 166 days and 199 days respectively, but the solar radiation will be in the ratios 63 :37. He puts the results as follows: Glacial 229 heat measures spread over 166 days.

136 heat measures spread over 199 days. Interglacial 229 heat measures spread over 199 days. 136 heat measures spread over 166 days.

E. P. Culverwell exposes the fallacy of taking an entire hemi sphere as a unit, and presents the maximum effects of winter in Aphelion at maximum eccentricity by comparing latitudes which at present receive a corresponding amount of sun-heat. A single illustration will suffice: Oxford, under the extremest conditions, would receive the same amount of sun-heat as Edinburgh receives now. A further objection to the astronomical theory consists in the fact that for each glacial and each interglacial period it pro vides only 10,500 years, which is certainly inadequate for the growth of an ice-sheet which shall extend from the Gulf of Bothnia to Norfolk—a distance of about 1,400 miles. Croll, who first elaborated Herschel's proposition, admitted the insufficiency of the hypothesis which would not diminish the total amount of sun-heat received annually by the hemisphere having its winter in Aphelion, and proposed several ways in which the effects of winter cold could be accentuated by fogs generated in summer over the snowfields by the chilling of moisture-laden winds. It is true that fogs do occur in the Arctic regions but, as has been pointed out by Wright, the heat thereby liberated raises the temperature of the snow and thus tends to neutralize the argu ment. Wallace, however, made the valuable suggestion that heat cannot be stored in the same way that cold in the form of snow can be stored, and that a snow field or ice-sheet tends to perpetu ate itself and so to carry over the effects from one precessional period to another though with fluctuations of the margins.

Croll and others have attempted to explain the Ice age or to supplement Croll's argument by modification of ocean currents, especially by the severance of the Isthmus of Panama, whereby the Gulf Stream would be deflected into the Pacific. To this hypothesis many objections can be urged, one of them quite fundamental, viz., that while it is true that the lowest part of the ridge dividing the two oceans is only 154ft. high, there is no evidence that it has ever been submerged since the Miocene period; on the contrary, the existing marine faunas of the two shores are so different as to indicate that there can have been no communication since about the middle of the Tertiary period. J. W. Gregory concludes that there is "no evidence, afforded either by stratigraphy or zoology, to show that the Atlantic and Pacific oceans have been united across Central America in post Miocene times." Even if a free communication had existed during the Glacial period it could have affected only that part of the Gulf Stream which emerges through the Straits of Florida, and not the equally important part which sweeps outside the Antilles.

Another agency invoked to reinforce Croll's argument was that the chilling of high latitudes in the Northern Hemisphere would increase the strength of the northern trade winds and cause a general displacement to the south of the equatorial current which feeds the Gulf Stream, whereby less warm water would pass into the north Atlantic circulation. This and other expedients seem necessary to supplement the astronomical theory, and when all is said, there remain strong grounds for doubting its sufficiency.

Telluric Causes.

Turning now to telluric causes—geographi cal and other—the first and simplest is the suggestion of Arrhenius that a decrease in the amount of carbonic dioxide in the atmos phere would cause a fall of temperature sufficient to produce a glacial period. Chamberlin has with a wealth of geological detail elaborated this hypothesis, but Angstrom has shown that the quantity of CO2 now present in the atmosphere absorbs all the radiations the gas is capable of intercepting.

Two hypotheses have been advanced having this in common that both would invoke elevation of the land as the primary cause of the Ice age, but they differ in that one would require epeiro genic uplift, i.e., uplift on a continental scale, while the other would find more local elevation sufficient, It is generally admitted that the chilling effect of altitude, other things being equal, will induce the formation of snow fields and glaciers so that no ques tion of adequacy need be raised, and discussion is confined to that of evidence. Both explanations derive some support from the existence of submarine prolongations of fjords or river val leys extending to great depths across the continental shelf. They have been recognized on many coasts not only in high latitudes, as off the coast of Norway, but at the mouth of the Adour, and at the entrance to the Straits of Gibraltar. Similar features charac terize the west coast of Africa where a submerged groove of great depth prolongs the course of the Congo. The Lightning channel, between the Faeroes and Iceland, furnishes another example and the whole Atlantic coast of North America from Labrador to the Gulf of Mexico, typically illustrated by the submerged troughs of the Delaware and Hudson traceable to a depth of 3,5ooft., have been cited as evidence of an uplift to nearly that amount in glacial times. No proof has been produced connecting this uplift, if it ever occurred, with the date of the Ice age, and Dana and other geologists have referred the erosion of these valleys to the Mesozoic period. The occurrence of dead shells of shallow water Mollusca over wide areas of sea-floor off the west coast of Norway down to 2,60o metres has been regarded by Nansen as proving an uplift of, at the least 8,000ft., but the facts would bear a quite different construction and may be inter preted as evidence of transport by floating ice.

Pre-glacial Land Levels.

It is useful in this connection to examine the evidence of preglacial land levels, and the records of events preceding the first indications of ice-action in the British Islands. Wright remarks that throughout the southern half of the British isles and the north of France the level of the sea in pre-glacial times was practically the same as at the present day. The shallow North sea, the English Channel, the Irish sea and many of the minor indentations, such as the Clyde and the Moray firth, existed then, much as they are to-day. A pre-glacial cliff with, in some instances, a beach at the foot, can be traced at intervals from Flamborough head southward to both sides of the Straits of Dover and along the south coast of England far up the coast of Wales, and along the south of Ireland. The Flam borough section is of particular importance, for on the beach which lies at its foot, a few erratics have been found, showing that some transport, perhaps by floating ice, was already taking place. An extensive denuded plain of marine erosion, having about the same slope as the existing sea floor, fronts the beach, and is traceable almost to the Wash. The sea, however, withdrew from the beach, which then became covered with landwash and blown sand, con taining mammalian remains including hippopotamus, and Elephas antiques, animals very characteristic, as already remarked, of the Chellean stage. After an interval, so brief that little or no erosion of the cliff took place, the whole was buried beneath the oldest of the three or four boulder-clays of the Yorkshire coast region. At the other side of the headland, a deposit with estuarine shells underlies the boulder-clay.

Before its full significance can be discussed, however, it is necessary to make a cast back to the immediately preceding geo logical period, the Pliocene. Of this period the geological record in the southern part of the North sea is singularly complete. It opens with the Coralline crag a rather shallow water accumulation con taining a fauna bearing a strong resemblance to that found at present on the coast of Spain and in the Mediterranean, but with a large infusion of elements at present found in the British sea and a very few forms now restricted to higher latitudes. Indi cations of floating ice are not wholly wanting. A slight upheaval brought this deposit above the sea, but it was again depressed to receive the Red crag, a series of beach or inshore deposits form ing a succession of beaches of a sea that retreated from south to north. The oldest of these deposits is at Walton-on-the-Naze where it occupies a position about 4of t. above sea-level. The newest is near Aldeburgh. In this series can be readily recognized the gradual substitution of a fauna very near to that of the Coralline crag, with its marked southern affinities, by one in which, along with many of the shells now found in the British seas, there is a large infusion of shells of high northern and some even of Arctic range. During all this period the delta of the Rhine on the opposite coast in Holland and Belgium was steadily sinking. Harmer has argued that the profusion of shells found in the beaches of the Red crag and the comparative scarcity on the east was the result of prevalent on shore easterly winds, and as an indication "that the climate of regions to the north of Great Britain had . . . by that time become considerably colder than now, and therefore frequently anticyclonic in winter, an ice-sheet having permanently established itself on the Scandinavian high lands." The further Pliocene history of the North sea is a story of steadily accentuating cold, subject to the qualification that a freshwater bed, the Forest bed proper, brings an anomalous warm flora and fauna into the midst of a series in which plants and animals alike bespeak a climate of great severity.

The next member of the sequence on the coast of Norfolk is the Cromer Till—a mass of boulder-clay of pronounced type, con taining, inter alia, erratics from the Oslo district of Norway. This Pliocene and Early Pleistocene record of progressive and appar ently uninterrupted refrigeration is a fact which seems to refute decisively the hypothesis that the glaciation was caused by con tinental uplift, and equally opposes the view that it was the effect of those astronomical causes invoked by Croll. The duration of Pliocene time must have greatly exceeded the possible duration of any progressive change of climate postulated by the theory.

Continental Uplift.

The hypothesis of continental uplift has received a new impulse in recent years by the recognition by La Mothe and Deperet of four successive raised beaches and river terraces at altitudes ranging from 3oof t. down to about 6oft. above the respective present levels of the Mediterranean, and the rivers discharging into it. The lower three of these have been considered to correspond with the last three interglacial episodes, but geological opinion is sharply divided as to the connection.

It is to be deprecated that in allusion to the events of the Ice age or to discoveries connected with the successive stages of primitive cultures free use is made of time scales for which little justification exists. The only definite measure at present available is that of de Geer and the various estimates of the age of the Falls of Niagara which give measures so far accordant as to make it fairly safe to assume that the glaciation of North America and Northern Europe was simultaneous. The hypothesis that would explain the great extensions of glaciers in almost every mountain centre in the world in recent geological time by sup . posing them to have been uplifted so as to produce a lowering of the snow line involves a series of coincidences not easy of accept ance, especially when synchronized in their advances and retreats.

The final disappearance of the ice-sheets of northern Europe and North America was attended by a rise of the centres whence they respectively emanated. The Scandinavian region, as is shown by raised beaches which on all the coasts show a steady increase of elevation as they are traced up the fjords, has risen since the departure of the ice to a maximum of over 800ft. which is attained, as according to the doctrine of isostasy it should be, not at the mountainous axis of the Scandinavian peninsula, but just where Ly other evidence it was proved that the maximum weight of ice rested. Near the outer margin of the ice sheet Scotland had been pressed down to an extent proportional to the weight imposed, and the recovery recorded in raised beaches is correspondingly less, whereas Scandinavia has not yet completed its recovery.

In North America, the shrinking Labrador sheet gradually withdrew behind the watershed between the Mississippi and the St. Lawrence drainage basins, and great lakes came into existence between the watershed and the retreating ice front. The strand lines or beaches rise towards the site of the ice-radiant in a manner analogous to the rise of the Scandinavian beaches. When the ice reached in its retreat to the north of Niagara the lake drainage fell over the steep escarpment and the famous Falls thus came into existence. The gorge of Niagara, 7m. in length, was produced by the cutting back of the Falls and estimates of the lengths of time occupied in the operation have furnished a measure of the date of this stage of retreat, but many complications are intro duced into the problem by the fact that at one stage an alternative route for much of the drainage was opened by way of the Mattawa, a tributary of the Ottawa river. There is a general agreement between the results obtained and those given by de Geer's measurements of the seasonal clays in Sweden. (P. F. K.)