Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

In addition, older landslides may have also occurred in this same area but may
have been obscured by the Pinedale-age landsliding. For example, the Bull Lake terminal
moraine would have been equally susceptible to failure while it was being deposited
(about 130,000-150,000 years ago) if a large earthquake had occurred during that time
period.
The other range-front landslide lies between the North Fork and Lost Creek and is
a failure of Proterozoic rock on the range-front faceted spur. This landslide deposit is
quite long in relation to its width, indicating that the landslide had enhanced mobility,
perhaps also an indicator of high water content.
The largest area of landsliding lies in Proterozoic rock on the south side of the
South Arkansas River, west of Maysville. The reach of the river valley west of Maysville
is by far the narrowest part of the valley, with the narrowest constriction between
Maysville and Lost Creek. Here, several segments of the southern valley wall appear to
have moved northward and nearly pinched off the valley. The youngest, lowest, and
smallest slide block is mapped as a landslide deposit (Qlso), is labeled Slide Block III,
and has a recognizable headscarp and somewhat hummocky topography. Roadcuts on the
north side of US 50 opposite Qlso expose extremely shattered and brecciated Proterozoic
rock that may represent a correlative part of Qlso now stranded on the opposite side of
the river. The landslide deposit is shaped like a triangle, bounded by linear flanks
trending northwest and northeast. These flanks parallel the trends of the two main fault
sets mapped south of the South Arkansas River. On the basis of this coincidence, we
believe that Qlso is a wedge failure of quasi-intact Proterozoic rock that is sliding
northward on the north-plunging intersection of NW- and NE-dipping normal faults.
Upslope from Slide Block III are two progressively larger areas interpreted as
older slide blocks (Slide Blocks I, II). Slide Block II is the lower and smaller block and
coincides with the narrowest, most constricted reach of the South Arkansas River. Like
Slide Block III, Slide Block II is bounded by linear northwest- and northeast-trending
gullies that parallel the two regional fault sets. Slide Block I is an even larger but less
well-defined block bounded by similar faults. Field checking revealed that the
Proterozoic rocks within Slide Blocks I and II are not generally any more fractured or
shattered than Proterozoic rocks beyond the blocks. This suggests that the slide blocks
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