344 B. WILLIS — LEWIS AND LIVINGSTON RANGES, MONTANA 



30 degrees northeast; in Flathead valley, drift and lake beds; in Liv- 

 ingston range, Appekunny argillite, dipping 30 to 45 degrees northeast. 

 The dip of the Siyeh formation in Galton range carries it under Flathead 

 valley to a position several thousand feet below the Appekunny beds, 

 which it properly overlies. The relations are those of a normal fault of 

 great displacement, and downthrow on the west. 



From the topographic relations the position of this normal fault is 

 inferred to be along the base of Livingston range, the downthrown block 

 underlying Flathead valley. 



Date of normal faulting. — By reasonable inference the lake beds of 

 North fork are connected with the normal faulting. The strata are 

 tilted to a dip of 14 degrees northeastward toward the fault, as though 

 they had shared in late movements. The date of faulting is thus tenta- 

 tively fixed as Miocene or possibly Pliocene. Elsewhere in Montana 

 similar faults are related to lake basins which have yielded Miocene 

 vertebrates and, pending exact determinations, the North Fork fault is 

 assigned to Miocene rather than Pliocene. 



This conclusion has been anticipated in placing the latest probable 

 date of Lewis thrust as mid-Tertiary; for the normal fault has resulted 

 in a detachment of Livingston range, such that the strata could not in 

 their present position receive the pressure which overthrust and flexed 

 them. It follows that the thrusting must have preceded the normal 

 faulting — that is, must have been accomplished by Miocene time. 



STRUCTURE AND PHYSIOGRAPHY 



Great plains and Front ranges. — Recognition of the tilted attitude of 

 Cretaceous strata and of the even surface extended across their edges is 

 sufficient to demonstrate the character of the Great plains, at least in the 

 belt adjacent to the Front ranges. The surface is one of planation, in- 

 dependent of structure, and, marine planation being excluded on strong 

 negative grounds, it may be considered a peneplain. Several stages of 

 erosion may be noted in the relief of the Great plains, but the one here 

 referred to is that which is represented by the highest levels and which 

 is oldest. In the preceding discussion of antecedents of the Lewis thrust 

 it was named Blackfoot peneplain and assigned to a pre-Miocene cycle 

 of erosion. It is well represented in Milk River ridge between Cutbank 

 creek and South fork of Milk river, west of the 113th meridian, where 

 its elevation above sea is between 5,000 and 5,100 feet.* 



The rise of the Lewis range above the Blackfoot plain is more than is 



♦See the Browning atlas sheet of the topographic map of the United States. 



