Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

the Dry Union Formation. These capping gravels produce a concentrated boulder lag that
drapes over and contaminates areas of underlying Dry Union Formation and hamper
recognition of outcrops. Observations are also hampered by the development of thick
soils and dense timber, which generally increase in thickness and density towards the
west and the Sawatch Range front. Consequently, the further west in the Dry Union
sequence the less understood the characteristics of the formation.
Ten orientations of bedding in the Upper Arkansas graben sequence (Td) were
found in the Maysville quadrangle. Nine are along the eastern margin of the quadrangle.
The orientations are variable over relatively short distances and the dip directions also
change. The dip of bedding ranges from 5 to 26 degrees and is predominantly west,
southwest, and northwest. Near the south end of the graben on the east side of the North
Fork, the orientation of bedding in a small area of badland pinnacles is north-south with a
12° west dip. These observations together with other bedding orientations from the Salida
West quadrangle indicate that the Dry Union Formation in the southern Upper Arkansas
graben does not have consistent bedding orientations and is probably warped by gentle
broad flexures or is locally deformed and rotated by faulting.
Subtle variations in the nature of the interbedded conglomerate lenses exist across
the piedmont area. North of Squaw Creek the conglomerate clasts are commonly up to 2
ft and locally up to 6 ft in size and consist of 40 to 60 percent Tertiary volcanic rocks
(flow rocks and minor welded tuff) and hypabyssal intrusive rocks, 10 to 25 percent
Mount Aetna quartz monzonite, 10 to 25 percent Mount Princeton quartz monzonite, and
0 to 7 percent Proterozoic rocks (Xgd and Xag). South of Squaw Creek the conglomerate
clasts are typically less than 6 inches and consist of 65 to 80 percent Proterozoic rocks
(Xgd and Xag), 10 to 15 percent Mount Princeton quartz monzonite, 5 to 10 percent
Mount Aetna quartz monzonite, and 0 to 5 percent Tertiary volcanics. These variations
are interpreted to be related to different levels of stratigraphic exposure with higher
proportions of Tertiary volcanic rocks, hypabyssal intrusions, and Mount Aetna ring
dikes at deeper stratigraphic levels of the Dry Union that record removal of the higher
levels (volcanic and hypabyssal rocks) of the caldera system. Higher stratigraphic levels
of Dry Union have predominantly Proterozoic rocks and Tertiary plutonic rocks. The
exposures of different stratigraphic levels of Dry Union Formation are interpreted to be
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