Geology . New Jersey were commingled with non-glacial wash-prod-ucts and will be discussed under the non-glacial formations (Columbia,p. 447). The Natchez formation.—At Natchez, Mississippi, there is a section of assortedmaterial about 200 feet in thickness which is chiefly made up of derivatives from 1 Leveret!, Mono. XLI, U. S. Geol. Survey, p. 235. THE PLE1ST0CEXE OR GLACIAL PERIOD. 387 the Lafayette formation, upon which it rests unconformably (Fig. 513); butit also contains crystalline pebbles and calcareous clays assignable to washfrom the glacial regions, all other assignments seeming t
Geology . New Jersey were commingled with non-glacial wash-prod-ucts and will be discussed under the non-glacial formations (Columbia,p. 447). The Natchez formation.—At Natchez, Mississippi, there is a section of assortedmaterial about 200 feet in thickness which is chiefly made up of derivatives from 1 Leveret!, Mono. XLI, U. S. Geol. Survey, p. 235. THE PLE1ST0CEXE OR GLACIAL PERIOD. 387 the Lafayette formation, upon which it rests unconformably (Fig. 513); butit also contains crystalline pebbles and calcareous clays assignable to washfrom the glacial regions, all other assignments seeming to be excluded by aspecial investigation. A marked interval between its deposition and that ofthe overlying loess is indicated. As the sub-Aftonian and Aftonian depositsare the only older ones with which great gravel deposits are known to be asso-ciated, and as the Natchez deposit must be referred to an early Pleistocene stagebecause the great Mississippi trench, 60 miles more or less in breadth, has been. Fig. 512.—Section just east of Oelwein, la. 1, Sub-Aftonian (Jerseyan); 2, Aftonian 3, Kansan; and 4, Iowan. excavated since it was formed, reference to one of these two stages is more plausi-ble than to any later one. This reference is strengthened by the fact that almostthe whole formation—which was clearly a valley train leading back to the driftarea—has been removed. Assuming the correctness of this reference and combining it with other data,the following tentative conception of the sub-Aftonian and Aftonian stages isreached. The ice-sheet spread from the Keewatin and Labradorean centers tothe approximate limit of the known drift in the Mississippi valley, and depositeda typical sheet of bowlder clay (sub-Aftonian) and also gave rise to great valleytrains of glacio-fluvial material that stretched from the drift border to the Gulf,filling the low-gradient valleys of the time to depths of 30 to 50 feet near thedrift border, and of 200 feet near the Gulf (Natchez form
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