VEGETATION AND SOILS. 449 



it follows along the eastern line of that county, passes into Portage, 

 curves westward, and crossing the Wisconsin river again, continues 

 in a nearly westward direction across Wood, Clark, Jackson, Trem- 

 pealeau and Buffalo counties, to about the foot of Lake Pepin, on the 

 Mississippi. The driftless region is thus nearly altogether within the 

 Silurian area. North and east of the drift limit the general contours 

 are usually flowing, the valleys frequently wide and ill defined, the 

 rock exposures not frequent, and the outliers not many in number, be- 

 ing usually of large size and without jagged edges or peaks. On the 

 other side of this line the topography is strikingly different; the 

 changes of level are abrupt, the valleys narrow, with steep and high 

 sides, the rock exposures frequent, and the outliers often of very small 

 dimensions horizontally, though of very considerable height, and of- 

 ten showing precipitous sides with jagged, peak-like summits, even 

 when of soft sandstone. The drift forces have contributed to this re- 

 sult in two M^ays; (1) by planing down the irregularities resulting 

 from subaerial erosions, and (2) by the deposition of great quantities 

 of clay, sand and gravel. The peculiar mode of deposition of this 

 material has, however, itself, in much of the region, produced a pe- 

 culiar irregularity of surface, leaving it covered with ridges of low 

 rounded knobs, and intervening rounded depressions, which are fre- 

 quently filled by small ponds or lakes. It is also true that the change 

 of topograph}' noted in i^assing from the driftless to the drift-bearing 

 area is not exclusively due to the former presence or absence of the 

 glacial forces, but is in some measure owing to the fact that for a 

 long distance the drift limit is nearly on the line of a change from a 

 horizontal position of the strata I0 a gradually increasing eastward 

 slope. 



A much closer connection can be traced between the variations in 

 surface features and the changes in the formations, which will receive 

 attention hereafter. 



VEGETATION AND SOILS. 



These are noticed here only in, their most general relations; the 

 soils only in reference to their connection with the underlying geo- 

 logical formations. 



The three different kinds of surface in the district, as to vegetation, 

 are the prairies, marshes and timber lanrl. The prairies, or tree- 

 less areas, are restricted to the Silurian region, and are chiefly charac- 

 teristic, in central Wisconsin at least, of limestone districts, though 

 occurring also on the sandstone formations. In many places regions 

 "Wis. SuR. — 29 



