384 GEOLOGY. 



I. The sub-Aftonian, or Jerseyan, glacial stage.— In Iowa there 

 is found a very old drift-sheet lying beneath the Kansan drift-sheet, 

 with sand and gravel, peat, old soil, and other products of an ancient 

 surface between them. It is not now known that this sub-Aftonian 

 drift-sheet comes to the surface, except as exposed by erosion, in Iowa 

 or other parts of the Keewatin area, and it is not yet certain whether 

 the oldest portions of the Labradorean drift are to be correlated with 

 it or not. It is reasonable enough in itself to believe that the earliest 

 ice invasion may not have pushed as far southward as a later one, 

 and such a view is held relative to the earliest glacial formation of 

 Europe. 1 In Pennsylvania 2 and New Jersey, 3 the frayed edge of 

 a very old sheet of drift emerges from beneath the much later drift 

 of the region, and this older drift may not improbably be the equiva- 

 lent of the sub-Aftonian of Iowa, but as direct connection cannot be 

 traced, the correlation is uncertain. 4 The sub-Aftonian is a typical 

 sheet of till notable for the relatively high percentage of its green- 

 stone erratics. It is exposed by erosion, or artificially, near Afton, 

 at Oelwein, and at other points in Iowa, and probably embraces nearly 

 all the sections of " lower till " cited by McGee in his paper on the 

 drift of northeastern Iowa. 5 



II. The Aftonian interglacial stage. — Overlying this till sheet at 

 many points is a stratum of sand and gravel, and at some points beds 

 of peat and muck, with stumps and branches of trees, together with 

 the physical indications of an interval of erosion and weathering. It 

 is not wholly clear whether the assorted drift constituted the glacio- 

 fluvial products of the closing stage of the sub-Aftonian ice epoch, 

 or was derived by secondary action from the drift during the inter- 

 glacial interval. In the typical localities between Afton and Thayer, 

 Iowa, the deposit contains bowlders of till, showing that it is truly 

 secondary, but this does not define its precise age. In some districts 

 assorted drift is sufficiently prevalent and continuous at this horizon 

 to give rise to local systems of flowing wells. Near the typical locali- 



1 Geikie's Ice Age, 3d ed., and Jour. Geol., Vol. Ill, p. 241. 



2 Williams, E., Proc. Am. Phil. Soc, Vol. XXXVII (1898), p. 84. 



3 Salisbury, Annual Report of State Geol. of N. J., 1893. 



4 The Albertan drift (province of Alberta, Can.), formerly thought to be the proba- 

 ble equivalent of the sub-Aftonian, is probably not of glacial origin. Calhoun, unpub- 

 lished data. 



6 Eleventh Ann. Report U. S. Geol. Survey. 



