TOURTELOTTE PARK SPECIAL MAP. 105 
‘limestone crops out, and the fault runs along the base of the scarp. In 
such a case as this, where the dislocation was accomplished mainly in pre- 
Glacial time, the erosive action of the glacier must have scooped out more 
of the soft shale than of the harder rock, and thus produced the scarp. It 
is to this differential glacial erosion that the formation of the entire hollow 
basin called Tourtelotte Park is due. 
As a general distinction between scarps of erosion and those of uplift 
in this region, it may be remembered that erosion scarps are likely to occur 
where there is considerable difference in hardness of the rocks on the two 
sides of the fault, and that in this case the scarp will probably be sloping. 
The uplift scarp, however, may originate perfectly well where the rocks 
have the same hardness on both sides, and such a scarp may have an 
almost perpendicular face. Faults undoubtedly exist where the movement 
was begun in pre-Glacial times, and still continues slowly, so that part of 
the movement is pre-Glacial and part post-Glacial; and along these may be 
formed scarps partly of erosion and partly of uplift. Pl. X shows the scarp 
of the Silver Bell fault, which is perhaps an illustration of this. The hill 
shown on the right side of the picture is made up of the hard limestones 
and dolomites of the Leadville formation, while the flat surface shown in 
the foreground is underlain by the soft black Weber shales. At the base 
of the hill is the fault, along which there is a zone where the dolomite is 
altered and silicified, so that it forms a sort of chert, which may be called 
jasperoid. The maim searp is undoubtedly due in this case to glacial 
erosion, but it is possible that a few feet of the cliff at the bottom, which 
may be found where not obscured by talus from the hill above, may be 
due to uplift since the Glacial period. This cliff is nowhere very strikingly 
developed, and in this picture can be seen only just to the right of the 
gallows frame in the center. 
RESUME OF STRUCTURE IN THE AREA OF THE TOURTELOTTE PARK 
SPECIAL MAP. 
The various mechanical accidents which have happened to the rocks 
in the Tourtelotte Park district since their formation, bringing about 
changes from their original position, may be enumerated as follows: 
1. The first deformation——The first deformation consisted in a folding of the 
sedimentary beds against the hard resisting granite axis of the Sawatch. 
