PLATE VIII. 



HOUSTON COUNTY, 1884. N. H. WINCHELL. 



Of all the southern counties this is the most hilly. It was channelled by 

 numerous water courses, and also suffered a general surface degradation, prior to the 

 spreading of the loam, and probably prior to the Cretaceous, i. e.,in the long interval 

 between the Lower Silurian and the Cretaceous, while the county and the surround- 

 ing country in Minnesota, Wisconsin and Iowa was probably a land surface. The 

 present rough surface is due to the excavation of channels by streams during that 

 long interval. How much of the eastern part of the county was originally covered 

 by the Lower Silurian can only be conjectured, but it seems to have been all covered 

 by a uniform stratum, or series, which was continuous with similar rocks on the 

 east side of the Mississippi in Wisconsin. 



The Mississippi and Root rivers are somewhat over 500 feet below the bluffs, 

 and all the smaller streams run in similar rock-bound gorges. The Root River valley 

 is about two miles in width between the bluffs, and the horizontal strata through 

 which the valley has been excavated present frequent outcrops in the form of 

 precipitous cliffs and crags. Away from the valleys the country is simply undulating 

 and sometimes spreads out in extended plateaus on which are large and flourishing 

 farms. 



The most elevated portion is about Spring Grove (1194 feet), where the elevation 

 is 574 feet above the Mississippi river at the southern line of the county. 



The soil of the upland country is generally rich, being made of the loess-loam, 

 both surface and subsoil. This loam is thicker and more clayey toward the Missis- 

 sippi, and occasionally becomes sandy in the western portion of the county. The 

 only place in the county at which a foreign drift was observed is at Riceford, where 

 a scanty foreign gravel was seen in the valley of Rice creek. A terrace largely com- 

 posed of similar materials is scantily exposed along the Mississippi. It can be seen 

 at La Crescent rising about 50 feet above the Mississippi flood plain. The lands in 

 the valleys are of alluvium, but sometimes are quite sandy. 



The rocks consist of alternating magnesian limestones and quartz sandstones 

 belonging to the Upper Cambrian, and of blue limestones and shales of the Lower 

 Silurian, the latter overlying the former and appearing in Wilmington and Spring 

 Grove townships. The bluffs of the Mississippi and Root rivers, and of nearly all 

 their tributaries, are sharpened along their crests by the Lower Magnesian limestone, 

 which is about 200 feet thick.* Above this stratum, but usually not well exposed, 



* This was wrongly called St. Lawrence limestone in volume i . The correction was made in the preface of volume ii. 



