66 PHYSICAL GEOGKAPHY. 



and three hundred feet thick. The valley has been eroded 

 entirely through both of these, and rnns the greater part of its 

 course within and upon the Trenton limestone. Thus all the 

 formations along and within this valley are Lower Silurian. 



That portion of the valley which traverses Fayette and 

 Clayton counties is a deep gorge, in comparison with the 

 valleys of the other rivers hitherto described, with steep and 

 often precipitous sides. High, rocky cliffs are not uncommon, 

 but usually the steep sides are covered with debris of suffi- 

 cient fineness to sustain a good growth of trees. The valley 

 is usually narrow, and without a well marked flood-plain. 

 Along a large part of its course the valley is too narrow to 

 contain much good farming land; and hitherto, it has been 

 found difficult to open a good wagon road along its entire 

 length. Its fall is so great that water-power is abundant, but 

 it is not accessible at all places, on account of the steepness 

 of the valley-sides. 



Upper Iowa river. There is frequent reason to complain 

 of the meagre nomenclature of the early pioneers as applied 

 to our rivers as before intimated. We have in consequence 

 of it two Iowa rivers with only the addition of the word 

 " upper" to distinguish the name of the one under discussion 

 from that of the one already described as Iowa river. The 

 Upper Iowa rises in Minnesota just beyond the northern 

 boundary of Iowa, but enters our borders before it has 

 attained any considerable size. Its course, although it has 

 many windings, is nearly eastward until it empties into the 

 Mississippi. It rises in the region occupied by Devonian 

 rocks, and flows across the outcrops respectively of the 

 Niagara, Galena, and Trenton limestones, the St. Peter's 

 sandstone, the Lower Maguesian limestone, and Potsdam 

 sandstone; into, and through all of which, except the last, it 

 has successively cut its valley. 



Its valley is the deepest of any in Iowa, reaching a depth in 

 its lower part of more than four hundred feet from the highest 

 ground in the vicinity. That portion of it which traverses 

 Allamakee county has the Potsdam sandstone composing the 



