10 



Farther to the west, the Tertiary formation immediately south of the Line has been 

 eroded to a depth of several hundred feet, and this has been carried out to such an extent 

 that isolated buttes have been left in otherwise broad plains. The whole area of about 8,000 

 square miles has been deeply scored by numerous intersecting valleys which make it an 

 almost impassable territory. These "Bad Lands" of the upper Missouri have been pushed 

 northward across the International Boundary where they meet the more undisturbed and 

 more elevated Tertiary plateau or Missouri c6teau. The drainage from this plateau south- 

 ward has resulted in the cutting of several great gorges or valleys as the streams descend 

 from the cdteau to the level of the Bad Lands, and in them important exposures of the for- 

 mation are to be met with. The most notable of these gorges are the Great valley at about 

 longitude 105", and Porcupine creek at about 105° 45' and thirty-five miles west of Pyramid 

 creek which may be considered in connexion with Great valley. These three regions— 

 Souris river, Great valley and Porcupine creek, although usually described separately, may 

 be treated floristically as one, for the reason that they represent exposures of the same for- 

 mation, and the floras are the same. 



GREAT VALLEY. 



The most complete account of the Great valley has been given by G. M. Dawson (<»,92), 

 who has shown it to be the most eastern great channel of erosion which crosses the Inter- 

 national Boundary southward, towards the Missouri, and in it the beds of Lignite Tertiary 

 are exhibited on a grand scale. They are exposed at an elevation of about seven hundred 

 feet above those of the Souris river at "Wood End,, and about six hundred feet above those 

 of the northern locality at Traders Road, but their exact relation to the latter has not been 

 determined. 



The complete section of the beds has a thickness of 210 feet, and the lowest of these 

 contain the plant remains. They consist of curiously banded clays and shales which have a 

 purple tint when viewed from a distance. In the upper portions of the section somewhat 

 abundant remains of moUusca occur but only those of fresh water origin. 



Many of the crumbling hill-tops of the valley have a brick red colour resembling that 

 seen in parts of the Souris valley, and due, as there, to the combustion in situ of the deposits 

 of lignite. The slag or clinker produced by this action is also found, though not observed in 

 place. 



PORCUPINE CREEK. 



Porcupine creek is the third of the great Tertiary exposures along the International 

 Boundary. In it and tributary valleys many partial sections occur (O, 97), and lignite is 

 seen in three places near the Boundary Line, just above the level of the water in each case. 

 The beds which overlie the lignite consist of yellowish and grey sands and clays, well strati- 

 fied, and closely resembling those forming the upper part of the section of Great valley. 



In a section thirty-one feet thick, the lignite bed lies within one to two feet of the bottom 

 and has a thickness of three to four feet. Above this, for twenty-five feet, there are alter- 

 nating strata of grey clay and sand, ranging from three inches to nine feet, which contain 

 plant remains in various states of preservation. 



