530 A. G. LEONARD 



recent years has shifted the margin somewhat farther west and 

 south and it is now provisionally located as shown on the accom- 

 panying map (Fig. o). 



Glacial drift is found 50 miles south of the mouth of the Yellow- 

 stone River, or within less than 15 miles of Glendive, Montana. 

 For a distance of 50 miles east of the western boundary of North 

 Dakota the drift margin extends approximately east and west and 

 lies 30 40 miles south of the Missouri River. North of the Kill- 

 deer Mountains the boundary swings sharply to the south, and 

 crossing the Knife River near the western edge of Dunn County 

 it takes a general southeasterly course across the state. The margin 

 of the drift is believed to cross the Northern Pacific Railroad and 

 the Heart River between 2 and 3 miles west of Gladstone. That 

 a lobe of the ice sheet crossed the Heart River at Gladstone is shown 

 by the presence of thick deposits of drift gravels on the upland 

 1-2 miles south of the Heart and at an elevation of between 100 

 and 200 feet above river-level. In places the gravel and sand have 

 a thickness of at least 90 feet, and the deposit contains a number 

 of good-sized granite bowlders. A well-defined gravel ridge 

 marks the edge of the drift for 3 or 4 miles in this area south of the 

 Heart River at Gladstone. This ridge rises 30-40 feet above the 

 surface on either side and falls away rather abruptly on the south, 

 while on the north the slope is more gradual. 



There is no evidence that the ice sheet extended more than 

 2 or 3 miles south of the railroad between Gladstone and Richard- 

 ton, but in this vicinity glacial gravel and a few small bowlders 

 occur that far south. Between the Cannon Ball and Heart rivers 

 glacial bowlders are found as far west as Elgin, or within 12 miles 

 of the western border of Morton County. In general the drift 

 margin between the Killdeer Mountains and the South Dakota 

 line lies from 40-60 miles west of the Missouri River. 



Thickness of the ice sheet. — In the vicinity of Berg in northeastern 

 McKenzie County there are twelve or fifteen high buttes, known 

 as the Blue Buttes. which are irregularly distributed over an area 

 of 15 or more square miles. Many glaeial bowlders occur on top 

 of these buttes at an elevation of over 2,700 feet above sea-level, 

 or 1,000 feet above the Missouri River onlv b miles to the east. 



