1:70 T. C. OHAMBERLIN — THE [NTERGLACIAL tNTERVAL. 



may be regarded a< glacial, although I think in some of the earlier reports 

 Archean pebbles were cited as an indication that these gravels were con- 

 temporaneous with the glacial deposits of the north. They have been criti- 

 cally examined during the summer by my colleague, Professor Salisburyi 

 ami during the entire season's Bearch he has not found a single pebble that 

 is referable to a glacial origin. Some years since 1 examined the same 

 formation with the like result. Professor Call has also examined some of 

 these deposits with a similar result.' The pebbles are chiefly of chert, and 

 were derived from the chert-bearing limestones, which are Largely Car- 

 boniferous, but reach as far down as the Lower Magnesian limestone. They 

 are, therefore, non-glacial. This is a matter of some importance, as th 

 Bands and gravels have not only been correlated with the glacial deposits, 

 but referred to the Cham plain epoch. They are very far removed from 

 the Champlain deposits in time, and that correlation is one of the great 

 errors of Quaternary geology. They are certainly preglacial in the sei 

 that they were not contemporaneous with the glacial incursion at its earliesl 

 maximum. They may have been contemporaneous with the very earliest 



_'-s of glaciation before the ice reached the Mississippi valley and was 

 able to mingle its deposit- with those of the valley. 



Now these gravels occupy a wide area stretching across the basin of the 

 lower Mississippi from some distance back in Tennessee, Kentucky, and 

 Mississippi to the high lands upon the Arkansas side, appearing in the iso- 

 lated upland called Crowley's ridge, which bisects the present bottom of the 

 Mississippi. The gravel stratum undoubtedly was originally horizontal, 

 but it now undulates more or less conformably with the surface. Theex- 

 planation of this, it seem- to me, is found in the gradual creep of the sofl 

 material of the hills as they were slowly carved out by erosion. The brows 

 of the hills in some cases have obviously crept down the slopes, for on the 

 summits we find the gravels compact and firm and the constituent pebbles 

 lying with their maximum diameters in a horizonal position, while the stratum 

 has level upper and lower bounding planes. On the slopes of the hills. 

 however, the gravel beds are more or less broken up and the pebbles have 



been disturbed and displaced and tumbled into various attitudes, such a- we 



mighl naturally expect under the hypotheses of a creeping movement on 

 the slope. It seems impossible to suppose thai this stratum of gravel was 

 originally deposited in the undulatory form in which it is now found. It 

 mighl be supposed thai the Bill which overlies this gravel bed was deposited 

 :i- a mantle ,, v ,r an undulatory Burface, but gravel does q >1 lend itself to 

 Buch a method of distribution. 



The overlying mantle, which now chums attention, consists of fine silt and 



embraces the loess deposits of the lower Mississippi. It Bpreads oul broadly 



ravel Btratum and extends Bomewhal beyond it, especially on the 



