PROVISIONAL HISTORY OF ORIGIN OF THE MISSOURI 491 



active and prevalent, and therefore their head erosion nmeh more rapid. 

 Eventually the lakes became less extensive, and as the ice withdrew 

 to the east again the lakes in general would become filled and drained 

 until they disappeared. The effect of the ice withdrawing to the east- 

 ward would be to uncover more lake area lying adjacent to the front 

 of the ice. 



The explanation of the larger streams lying not far from the perma- 

 nent and larger moraine, known as the Altamont, is to be found in the 

 principle already mentioned, that the head erosion is particularly active 

 in the streams immediately supplying water from the ice. This explains 

 wh}^ the present course of the Missouri was located quite early. This 

 became permanent before the withdrawal of the ice from the Altamont 

 moraine. In short, the waters from the moraine eroded quite rapidly, 

 because the coarse material was dropped next the ice, so that erosion was 

 accelerated at a short distance outside of the main deposition. We con- 

 ceive of the Missouri cutting down deeper and deeper during the Pleisto- 

 cene, excavating what Dr. Leonard has called "the pregiacial valley.*' 

 This reminds us of a point of which we have said little, but which in its 

 combined effect serves as a powerful argument for the similar origin of 

 the Missouri in all three States under consideration. 



TERRACES 



In the valleys of the Missouri and its principal tributaries, there are 

 many very striking river terraces. These terraces are mainly the result 

 of erosion, the lower part of them being composed of bedrock and the top 

 composed largely of coarse material, mainly from glacial drift, the finer 

 silty material being largely washed away. A general statement applying 

 to all these terraces may be given as follows : 



They vary in altitude above the principal stream from 20 to 450 feet. 

 Those lower than 20 to 40 feet show few boulders and are knoA^ai as "silt 

 terraces" or "second bottom." They probably mark the erosion which 

 has taken place since the disappearance of the ice. Those above 100 feet 

 are usually largely composed of boulders in the upper portion. These 

 boulders are the larger and more conspicuous, from 200 to 300 feet. 

 These upper ones are called "boulder terraces," and without doubt mark 

 the intensity and altitude of erosion at different stages in the early Wis- 

 consin stage. These terraces are as near level as usual mider such cir- 

 cumstances, except for several miles below Bijou Hills, where some of 

 the terraces show a much steeper slope down the stream. This uniform 

 arrangement corroborates more strongly the idea that the history of the 

 streams in these three States have been unusually uniform. Not only 



