THE NORTHERN PACIFIC ROUTE. 103 
pressure ‘from the north so great that the rocks have broken, as 
shown in the diagram, and the Paleozoic formations on the north 
have been pushed over the Cretaceous formations on the south. Such 
a break is called an overthrust fault. 
The main part of the arch visible from the train is composed of 
Madison limestone, which is first seen from a westbound train at 
a point between mileposts 25 and 26. As shown in the diagram, the 
limestone at this place is only the broken fragment of the south side 
of the fold which has been nearly removed by the fault that crosses 
at the small loading platform on the right. The limestone at this 
place is overturned, dipping to the north, and it is probable that if 
it could be followed downward it would be found to grow thinner 
(beveled) and finally to be cut off altogether by the fault. 
At milepost 26 the Madison limestone gives place to the under- 
lying Threeforks shale (Devonian). The upper shaly part of this 
formation is covered so that it can scarcely be seen, but the lower 
part, consisting of a conglomerate composed largely: of red granite 
débris, is seen in rugged cliffs on the east side of the track. 
In the high mountain on the right there is said to be a wonderful 
cave in the Madison limestone, which has not yet been thoroughly 
explored. A quarter section (160 acres) of land, including the cave, 
was set aside by President Roosevelt on May 11, 1908, as the Lewis 
and Clark Cavern National Monument. The cavern, however, is so 
nearly inaccessible that it has been visited by only a few of the more 
hardy travelers. 
Opposite Lime Spur, 8 miles west of Sappington, the beds of 
Madison limestone outcrop along the canyon. wall like great white 
ribs, making some of the most rugged scenery to be 
found on the road. The limestone is quarried exten- 
sively at this place and shipped for the manufacture 
of cement. 
West of Lime Spur the Madison limestone gives place to the Quad- 
rant formation, which is well marked by the bright-red color of some 
of the limestone and shale beds that it contains. Beyond this band 
of bright color is a small’ exposure of Jurassic rocks which are cut 
off by an immense overthrust fault, shown in figure 20. This fault 
separates the rocks already described from conglomerate and argillite 
Lime Spur. 
Elevation, 4,260 feet. 
St. Paul, 1,084 miles. 
west of Sappington, could have been pro- 
duced only by folding and faulting, which 
were the results of strong forces that 
crowded the rocky layers into this region 
from both the north and the south. This 
caused them to buckle along east-west 
_ much as a pad of paper will buckle 
y the hands along two 
pee edges and feanetae hy The 
. 
great compression which affected the 
rocks in this canyon produced at least 
three scien folds or anticlines, and as 
the ent was strongest from the 
north the "folds were pushed over toward 
the south. As the pressure popes the 
rocky folds were broken, crushed, 
crowded together into the positions ot 
on the west side of the river. 
