102 PHYSICAL GEOGRAPHY. 



of tlie Missouri river. The stratification there is sometimes 

 very distinct, and in some instances the limy water that 

 issues upon it from the Bluff Deposit has so cemented the 

 sand and gravel together that it has thereby produced real, 

 although worthless, sandstone and conglomerate, the beds 

 of which are so regular as to have caused many persons to 

 mistake them for outcrops of regular ledges of rock. These 

 examples of Altered Drift along the bluffs of the Missouri 

 river are unusually distinct. 



Other examples of the more slightly Altered Drift may be 

 seen in the valleys of almost any of the rivers of the State 

 that have attained considerable size. They are particularly 

 observable in those valleys that have been eroded out of the 

 drift alone, such as those of Rock river and the upper part 

 of the Little Sioux; those of the upper branches of the Des 

 Moines, etc., the bottoms of which to a great extent do not 

 consist of Alluvium, properly so-called, but of Altered Drift, 

 often only slightly modified. 



Another phase of the Altered Drift, as included in these 

 descriptions, may be seen near the mouths and confluence of 

 rivers. Here the alteration is greater, and approaches the 

 alluvium in character. Sometimes upon the sides of valleys 

 the Altered Drift assumes somewhat the form of river terraces, 

 but they are really quite different in composition. The river 

 terraces are always composed of finer materials, and are 

 always free from boulders, while the others may contain both 

 gravel and boulders in comparative abundance. 



3. ALLUVIUM. 



The deposit here designated as Alluvium is that which has 

 accumulated in the valleys of rivers by the action of their 

 own currents. The materials composing it are all derived 

 from the rocks or deposits out of which the rivers have eroded 

 their valleys, and consequently it has all been transported to 

 a greater or less distance by their waters. It is this deposit 

 that always constitutes the flood-plains and deltas of our 

 rivers, as well as some of the terraces of their valleys. Some 



