June 29, 1888.] 



SCIENTIFIC NEWS. 



605 



THE DRIFTLESS AREA OF THE UPPER 

 MISSISSIPPI. 



PERHAPS some reader may require to be reminded 

 that by the term " f/r//7" geologists mean a layer 

 ot" large and small blocks of stone, angular or rounded, 

 mixed with sand or clay, heaped irregularly over the 

 other rocks, and to be traced over much of Northern 

 Europe and North America. 



Now among the many interesting results of the United 

 States Geological Survey, one of the most striking is the 

 discovery of a tract of about 10,000 square miles in the 

 basin of the Upper Mississippi free from the drift 

 This island, or oasis as we may call it, is most singular 

 in its character. To find some mighty mountain, which, 

 had towered above the great sheet of ice which over- 

 spread the region in the glacial age, and which had thus 

 escaped the deposit of drift, would create no surprise. 

 Nor should we think it strange if a table-land near the 

 border of the area of drift had stood out above the in- 

 vading ice. But here we find a broad tract, lying in the 

 very valley of the chief river of the region, overlooked 

 by higher land on different sides, and 3'et remaining un- 

 touched by the glaciation. What is most curious is that 

 the margin of the drift, nearly on all sides, lies on a slope 

 descending towards the driftless region. The drift-bear- 

 ing ice was checked in its course-, not by some steep 

 ascent which it could not surmount, not by some great 

 structural barrier, but by some difterent agency which 

 arrested it on its downward course. Arovmd it spread 

 the drift to the extent of 340 miles to the west and 

 south, and to a greater distance northwards and east- 

 wards. 



The driftless territory is rudely four-sided in its figure, 

 and it extends over several distinct geological forma- 

 tions. As compared with the adjacent drift-covered 

 regions, this area is remarkable for the absence of water- 

 falls and the rarity of constricted gorges. One ot the 

 most decisive proofs that the region has never been in- 

 vaded by glaciers, is to be found in the fragile pinnacles 

 of rock which abound over the northern portion of the 

 district. It is inconceivable that a great ice field should 

 have swept ouer the region wilhout rounding off the 

 rocks or entirely overwhelming the more slender forms. 



The principal features in the history of this driftless 

 area seems to have been an extension of the glaciating 

 agencies from the north around both sides, and their re- 

 junction below it. The second event would be the par- 

 tial deglaciation of the region to such an extent as to 

 permit the growth of vegetation over its surface. Not 

 indeed that it was a dense forest region, but it was still 

 clothed with trees. 



The third event was a reglaciation, which spread over 

 Northern America, a covering of ice even deeper than 

 that in the first stage. Then followed a long period 

 of freedom, from glaciation, giving place to a fresh inroad 

 of ice. After the cessation of the glacial inundations, 

 the rivers carved the diluvial plains into terraces. 



We have now to raise the main question, i.e., how the 

 driftless area escaped the repeated glaciations which 

 desolated the surrounding country ? The very fact that 

 the glaciation of the surrounding country was repeated is 

 very significant, proving that the immunity of this area 

 was no mere fortuitous incident. The cause must have 

 been a constant one, warding off each successive attack 

 of the ice-fiend. 



It cannot, as we have seen, lie in the elevation of the 



area, unless it was daring the glacial period of a de- 

 cidedly greater altitude than at present, for which there 

 is no evidence. 



As far back as 1877 Professor Winchell pointed out 

 that the driftless area lies in the lee of the elevated terri- 

 tory of Northern Wisconsin and Michigan, which, he 

 maintained, acted as a wedge, forcing the ice aside, and 

 so protecting the driftless portion in its rear. Professor 

 Irving, about the same time, argued that the great valleys 

 of Lake Superior and Lake Michigan lie in such a posi- 

 tion with respect to the driftless area as to divert the ice 

 streams to the right and left, and that this, in connection 

 with the highlands lying between the lakes, turned the 

 ice away. Mr. Strong, on investigating the lead region 

 in the centre of the district, advanced a similar opinion. 



But these considerations scarcely meet all the difficul- 

 ties ot the case. Other causes, not as yet distinctly 

 formulated, must have come into play. 



It remains to inquire whether, in addition to the topo- 

 graphic influences, climatic causes may have come into 

 play ? Were there any special climatic influences which 

 favoured the wasting of the advancing ice currents ? 

 None have been traced as specially inherent in the 

 region. Neither the rainfall, the cloudiness, nor the 

 winds are notably greater or less in this area than in the 

 adjacent ones. Climatic charts show local variations, 

 but these are not conterminous with the area in ques- 

 tion, nor with the ivaslage area in which the threatening 

 glaciers were consumed. 



Messrs. T. C, Chamberlin and R. D. Salisbury, the 

 geological surveyors, remark that the law of the self- 

 perpetuation of climatic conditions was doubtless effec- 

 tive. Just as there can be little doubt that the ice-clad 

 regions increased the snow-fall upon themselves, and 

 tended to self-perpetuation, so the ice-free area by 

 warmth, and therefore relative drought brought back 

 the advancing ice. At the time when it was most 

 threatened — when the ice seems to have completely 

 surrounded it — a special action may have come to its aid. 

 The prevailing winds, coming as at present from the 

 south-west, as they encountered the ice-belt on the west 

 and south of the area, were freed from their moisture by 

 the cooling effects of the ice and of their ascent of 

 the western slopes of the glacier. As they descended 

 on the east side they regained their capacity for 

 moisture, and became dry and absorbing winds, and 

 the extent of the area may not have been sufticient to 

 saturate them again before they encountered the ice 

 upon the southern or e^istern frontier. But the explorers 

 hold that the primary causes were topographic, climatic 

 agencies plaj'ing merely an auxiliary part. 

 ♦^>»^<^5«f-» ■ 



Influence of Seasons upon Vegetation. — According 

 to Le Voltaire, last year the buds of the chestnut opened 

 on April 19th. This year they have appeared on the 

 2oth, their mean being the isth. For their flowers and 

 those of the lilac to open, tfie mean temperature must 

 have reached 53° Fahr., and the maximum C^;^'' Fahr. 

 Wheat, in order to spring up, requires an accumulated 

 temperature of 302" Fahr. ; to produce ears, 2,426' Fahr.; 

 and to ripen, 4,000" Fahr. Barley makes lower demands, 

 an accumulation of 3,270'' being sufficient to bring it to 

 maturity. The vine deinands 5,250° Fahr., setting out 

 from so'' Fahr. as a lower limit. The date tree require", 

 to ripen its fruit, an accumulated temperature of 9,000° 

 Fahr. Butterflies do not issue from the chrysalis until 

 the mean temperature exreeds 58° Fahr. 



