8 The American Geologist. January, 1895 
sheet, oxidation, leaching, soil production, and a general de- 
gradation of the surface of the drift, with a buried forest bed, 
which occurs only locally in Illinois but is luoregenerall}^ dis- 
tributed on the Iowa side of the Mississippi river. This pe- 
riod of subaerial erosion was followed, in northwestern Illi- 
nois, by a period of loess deposition. As it is held by some 
geologists* that the Columbia formation dates from the first 
ice invasion of eastern North America, with a continuance of 
its deposition somewhat later, and also that the loess of the 
central and southern Mississippi valley is a portion of this 
formation, some dovibt has been expressed that the loess of 
northwestern Illinois is the chronologic equivalent of that of 
the lower Mississippi valley. It is the aim of the present 
paper to show that it is so equivalent: also that there is a re- 
markable parallelism between the several members of the Co- 
lumbia in the Mississippi embayment and in portions of 
nortliern Illinois. 
In the topographic basin of the Pecatonica river, in the 
north central portion of this district, there is a more complete 
and more easily deciphered record of the stratal succession 
than in any other portion, and my remarks will, therefore, re- 
late chiefl}^ to that subdistrict. It is mostly comprised within 
the tract of early drift adjoining the southeastern part of the 
Wisconsin driftless area.f 
The Columbia formation in this part of Illinois is clearly 
differentiated into three distinct members, which, for conven- 
ience in discussion, are designated respective!}' as the Fkirence 
gravel or Fluvial member, the Vallej^ loess, and the Upland 
loess. These will be described under their separate headings, 
with discussion of their mode of formation and significance. 
Florence Gravel. 
This, the basal member of the Columbia deposits, is con- 
fined to the lowest levels of the principal valleys, and its out- 
*Twelfth Annual Report of t lie U. S. Geol. Survey, lor ]890-'91, Part 
I. p. 402. 
fConsult Prof. T. C. Chamber]iii'.s map of tiie drift-bearin<? areas of 
the United States. formin<i' plate viii in tlu' Seventh An. Rep., U. S. 
Geol. Survej-, for the relationship of the Pecatonica basin to the earlier 
and later drift and to the driftless area of Wisconsin and the northwest- 
ern corner of Illinois. For maps of larger scale showing the basin, with 
the prominent moraines on the north and east and tlie driftless area on 
the northwest, set' plates x.\ix and xxxi in the Third An. Rej)., and 
plate xxvii in the Sixth An. Rep., U. S. Geol. Sur\c\ . 
