EFFECTS OF LOESS ON STREAMS ' 247 



ute the relatively slight slope of the detrital cones in this arid district, 

 which often have a declivity of no more than 30 or 40 feet to the mile. 

 This small measure of slope is perhaps in part due to the fine silt which 

 they obtain from the loess deposits which are continually forming in the 

 fields adjacent to their beds. Owing to this overburden of sediment, the 

 scouring power of the water becomes ver}^ small. 



The exceedingl}^ muddy character of the waters of the Missouri may 

 to a great extent be explained by the above noted peculiarity in the de- 

 livery of fine silt to its stream. As in other rivers which drain rather 

 arid districts, the torrents and even the main branches of the greater 

 streams are very extensively supplied with wind-blown detritus. In fact, 

 the supply of fine debris thus brought to the streams is much greater 

 than would be afforded them in a humid region, in which there was no 

 wind erosion and where the only seat of its production was the narrow 

 zone of the torrent beds. In this arid district of Montana the delivery 

 of finely divided rock-waste goes on quite as effectively as it does in ordi- 

 nary humid countries, for the reason that in the time of melting snows 

 and of torrential showers (the so-called cloud-bursts) the torrents are as 

 numerous and for a time as efficient in cutting and carrying pulverized 

 rock as they are in any district I have examined. To this normal 

 silt-making action is here superadded that brought about by the dust 

 which has been swept by the wind to the strip of humid and therefore 

 vegetative ground. In this position the stream, in its normal swing- 

 ings, is sure, within a limited time, to erode it. In the manner above 

 noted it comes about that a very large part of the finely divided rock 

 matter of this arid district, though not produced in the normal manner 

 by the torrents, is carried through the air to the lower reaches in the 

 broad valleys, and is there held by vegetation until it is taken up by the 

 stream and borne to the main rivers. A portion of this cordilleran dust 

 apparently escapes from the mountain region and is deposited on the 

 plains to the eastward, there also in places mainly adjacent to the rivers, 

 where it is likely to be taken up by their water. 



Stratigraphic Rklatiokships and Age of the Loess 



The most important geological indications afforded by the loess deposits 

 of the cordilleran valleys are derived from the relation of the beds to 

 those oil which they lie. Wherever I have seen this contact it is toler- 

 ably sharp. In most instances the underlying material is coarse sand 

 or fine gravel, which rapidly, within 10 feet or less of depth, verges into 

 a pebbly or bouldery bed. These deposits do not contain any distinct 



