A FRA C TURE VALLEY SYS TEM 9 7 



A simplified drainage map in which the drainage lines are some- 

 what straightened is given on the thin sheet, which is intended to 

 be placed over the topographic map. The purpose of straight- 

 ening the drainage lines is to present a simpler expression of 

 the system, rendering the persistency of some of the directions 

 more evident, and permitting the relationships of the various 

 directions to be more easily noted. It does not follow that 

 drainage channels would be more in accord with fracture systems 

 if they were straighter. Fractures and faults are not necessarily 

 straight. They are more often curved or crooked, as is the case 

 with most of those observed in the region under discussion. The 

 fault lines and dikes are printed in blue on the thin sheet. It 

 is probable that the irregularities of direction in the drainage^ 

 as it is drawn on the topographic map, are more in accord 

 with the fractures than the straightened lines traced on the 

 drainage map. The value of the tracing is in the simpler 

 expression of the system, serving the same purpose as a simpli- 

 fied statement of a highly complex set of relationships. It may be 

 looked upon as a diagrammatic statement of the drainage system. 



The region embraced by the map lies between the meridians 

 110° and 1 1 1°, and latitudes 45° and 46°. It is immediately 

 north of the Yellowstone National Park, and contains the Snowy 

 Range, the eastern slopes of the northern portion of the Gallatin 

 Range, part of the Bridger Range, and the southern slopes of 

 the Crazy Mountains. The Yellowstone River traverses the 

 quadrangle from the south and southwest to the northeast, its 

 tributaries intersecting the country in all directions. 



A study of the topographic map reveals the angular char- 

 acter of much of the drainage system, and the prevalence of 

 certain parallel and sub-parallel lines which appear in various 

 streams and occur in quite diverse portions of their channels. 

 Along parallel lines different streams may be flowing in opposite 

 directions ; one stream may be near its source, another near its 

 mouth, having other portions of their channels trending in other 

 directions. The persistency of these lines becomes more striking 

 when the geological structure of the region is taken into account 

 and it is observed that certain drainage lines traverse rocks of 



