452 GEOLOGICAL EXCURSION TO THE ROCKY MOUNTAINS. 



This broad valley represents one of a plexus of channels tunned by the 

 Hoods derived from the melting toe about the close of the Pliocene: it 

 is the Mur<iis WOgee of the French missionaries, which, traced against 

 the general direction of river How, extends northeastward and then 

 northward for 40 miles, where it divides once more, this time into three 

 branches. The Smallest of the branches coincides with the present 

 course of the Mississippi about Clinton; the next in size coincides 

 with the Wapsipinnicon River for 50 miles; and the principal branch 

 diverges from the Wapsipinnicon a few miles above its mouth and then 

 cuts directly across the interior of eastern Iowa to reunite with the 

 Mississippi at the mouth of the Maquoketa, 100 miles (by river) above 

 Rock Island. 



Bast of the plexus of ancient channels the road traverses one of the 

 low divides characteristic of northern-central Illinois — a typical prairie 

 land. On approaching Illinois river the surface becomes more rugose, 

 and rock outcrops appear. About La Salle and Ottawa the river bluffs 

 are 100 to 200 feet high, and expose a variety of formations within a 

 limited area, ranging from the coal-bearing Carboniferous through the 

 Niagara (the characteristic Upper Silurian formation of the interior) 

 and down to the Oneota (or "Lower Magnesian)," or the St. Peter. In 

 this vicinity the structure is more complex than elsewhere in Illinois 

 save in two localities, i. e., at the mouth of Illinois river, and near the 

 continence of the Ohio; it is a region of decided deformation of uncon- 

 formable strata, afterward planed down to base level. Noteworthy 

 fossil localities occur in the vicinity. The rocks are mantled with 

 glacial drift so deeply that exposures are rare, except in the river bluffs. 



Nearly all the way from Ottawa to Chicago the route traverses monot- 

 onous prairie land, faintly relieved here and there by moraines, incon- 

 spicuous in comparison with those of the northern route. There are 

 few rock exposures, the most notable being at Joliet, where there are 

 extensive quarries. The subterrane from Ottawa to Chicago is deeply 

 buried beneath the drift, but is probably almost wholly Upper Silurian, 

 and chietiy the interior represensative of the New York Niagara. 



