BLACKFOOT PENEPLAIN AND EROSION CYCLES 535 



from the higher level, declines northeastward 300 feet in a distance of 

 iy 2 miles. Beyond this for a distance of 19 miles the remarkably uni- 

 form, flat surface declines northeastward at an average rate of about 50 

 feet per mile. One traversing this gently sloping, second plain sees near 

 the middle of the east boundary of the Browning quadrangle, in town- 

 ship 35 north, range 11 west, the remnant of the higher level standing 

 with abrupt marginal slopes and flat top 100 feet or more above the 

 second plain on which he stands. The relations are illustrated in figure 2. 

 Small remnants of this upper level are also found on the highest parts of 

 Horsethief Eidge and Landslide Butte, 15 to 19 miles farther east. Simi- 

 lar relations of a higher level and a second lower level are found on the 

 ridge between the South Fork and Middle Fork of Milk River, and also 

 on the ridge between Middle and North Forks of Milk River. The rela- 

 tions are such as to indicate that following the development of the highest 

 plain, the Blackfoot peneplain, a moderate elevation of the region oc- 

 curred during which this first plain was dissected. The elevation prob- 

 ably was not great and the stream gradients were low; so that broad, 

 shallow valleys were eroded, developing the second set of smooth, flat, 

 gently sloping plains. The directions of slope of the remnants of the 

 highest plain are not in all places clearly toward present drainage lines. 

 Those of the second set, however, are toward the present drainage lines, 

 indicating that the main streams of the present system are following the 

 lines of those of the second stage, but at relatively lower levels. 



Following this stage of erosion renewed uplift caused dissection of the 

 second set of plains and the development of the broad, open valleys, into 

 the bottoms of which the present streams have incised their channels. 

 The third set of smooth, gently sloping valley plains, those developed at 

 this stage, are represented by the little dissected plain of the North Fork 

 of Milk River, the more dissected valley slopes of Middle Fork, Livermore 

 Creek, and South Fork of Milk River, the plain between Greasewood and 

 Cutbank creeks and by Carlow Flat. 



Wisconsin Stage of Glactation and the third Set of Plains 



The development of the third set of plains was followed by the Wis- 

 consin stage of glaciation. At (his stage Saint Mary Glacier was diverted 

 northward down the great open trough. Tn the vicinity of Duck and 

 Goose lakes the ice overtopped Saint Mary Ridge and spread eastward 

 onto the head of the North Fork plain. Cutbank Glacier extended down 

 Cutbank Val ley nearly 12 miles beyond the mountain front onto the plain 

 between Greasewood and Cutbank creeks, ami a small lobe occupying the 



