SOURCE OF THE SPRINGS 345 



of the rocks in this district is north, away from the mountains, to\vai-I 

 the plains, but local folding of the strata and erosion on a line between 

 Giant springs and high mountains to the soutliward exposes beds down 

 to a point about 150 feet below the top of the Madison limestone, thus 

 affording a vent for any water tliat may accumulate in the upper part of 

 the Madison or in beds overlying throughout their distribution in the 

 higher portions of the Little Belt mountains. AVhile no special examina- 

 tion has been made of the water capacity of the lower part of the Madison 

 limestone, it is believed from a general study of the occurrence of under- 

 ground waters in this district that it is not a good water bearer. 



It has been believed for some time by a few local observers that the 

 water of Giant springs comes from Gibson flat, where a strong underflow 

 is known, to exist. Most of these observers are of the opinion, however, 

 that the original source of this water is in Little Belt mountains, and 

 that it comes from that district as an underflow by way of Sand coulee 

 and its various tributaries into Gibson flat, rather than down the pre- 

 glacial channel of Missouri river from the present stream valley. That 

 the amount of underflow in the upper part of Sand coulee and its com- 

 Inned branches is small and entirely inadequate to supply the Giant 

 springs is shown by a test which was made of the underflow in one of the 

 largest tributaries of this drainage at the town of Stockett. Here a well 

 was dug in the middle of the valley by one of the coal companies, through 

 alluvium to bedrock, and from the bottom of the well tunnels were exca- 

 vated in either direction to the sides of the valley in order to catch the 

 entire underflow. The maximum capacity of this well is about 125,000 

 gallons per day, while that of the Giant springs is over 400,000,000. 



From a careful study of the geologic relations in the vicinity of Great 

 Falls, it is believed by the writer that the water of Giant springs is de- 

 rived from the subriver bed of the Missouri, leaving that valley near the 

 mouth of Sand coulee as an underflow and passing down the preglacial 

 channel of the Missouri, which is in reality up Sand coulee, into Gibson 

 flat, an oxbow in the old river channel. From here, by a subterranean 

 passage through porous Cretaceous sandstone and sandy shale, which dip 

 in a favorable direction for its transmission, it makes its escape to the 

 present Missouri river, where it appears in the form of the Giant springs 

 (see plate 20). 



It is further believed by the writer that the jointing, which is here well 

 developed with the major joint planes extending in a north-south direc- 

 tion, is an important factor in the underground movement of the water. 

 It is also possiljle that a fault in this vicinity further facilitates the 



