of dikes (molten rock injected into vertical cracks) stand out 

 from the surrounding sandstone cliffs. The dikes themselves are 

 much more resistant to weathering than the surrounding sandstone. 



As the Missouri River nears the confluence of the Judith 

 River, it cuts deeper through the layers of rock, and the 

 sandstone gives way to weathered shales. This steep, rolling 

 terrain, referred to as the Missouri River Breaks, is adjacent to 

 the flat valley floor and extends downstream to Fort Peck Dam. 



The Judith River originates in the Little Belt Mountains in 

 central Montana. One of the better known features in this area is 

 Yogo Gulch, home of the Yogo sapphire, noted for its brilliant 

 blue color. The sapphires have eroded from an igneous dike, and 

 the diggings have disturbed some of the area. 



East of Lewistown, paleontologists have discovered fossils of 

 soft-bodied fish. In most fossils, the shape of shells or bones 

 have been preserved. The site outside Lewistown is noteworthy 

 because it contains fossils of primitive shark-like fish that had 

 cartilage instead of bones. Other soft-bodied animals have been 

 preserved in this area as well. Some of the small sharks are only 

 two or three inches long and are preserved nearly perfectly. 



Another interesting feature southeast of Great Falls is the 

 old channel of the Missouri River. As the continental ice sheet 

 moved south during the last ice age, it diverted the Missouri 

 through what is now a portion of Shonkin Creek. There was even an 

 impressive waterfall on the old river. The rim of the now dry 

 falls can be seen at Lost Lake, which formed in the plunge pool of 

 the falls. About four miles south of Geraldine is the Shonkin Sag 

 laccolith, a classic example of an exposed igneous intrusion with 

 contrasting rock types resulting from separation of non-mixable 

 molten rocks. The laccolith covers over 1,000 acres, but is best 

 exposed in the vertical cliffs on its south face. 



The lower portion of the Musselshell River was also pushed 

 south by the glaciers. The present channel of Beaver Creek was 

 probably occupied by the Musselshell River in pre-glacial times. 

 It is not too difficult to trace the former Musselshell course 

 northward from U. L. Bend and Fort Peck Reservoir. 



The Smith River 



The Smith River, known for its quality trout fishing and 

 spectacular limestone canyon, comprises a unique river system for 

 central Montana. Originating within a high intermountain valley 

 between the Castles, Big Belts and Little Belt Mountains, the 

 Smith gradually cuts its way down to the Missouri. The river 

 flows north for approximately 126 miles through a valley sparsely 

 populated with small agricultural communities. The Smith River 

 Canyon, the river's most well-known feature, begins about 90 miles 



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