Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

on U.S. Forest Service color aerial photographs (1:24,000 scale) taken in September
1997. The annotated photos were scanned and imported into ERDAS Imagine Stereo
Analyst where they were photogrammetrically corrected and rendered in 3-D by
Colorado Geological Survey personnel. Line work was digitized directly from ERDAS
Imagine Stereo Analyst (by Shannon) and exported as ESRI shapefiles. Geological
editing and final map production was completed by Tom Neer at Digital Data Services,
Inc., Lakewood, Colorado.
The Quaternary deposits of the Maysville quadrangle are generally not well
exposed, due to the lack of artificial and natural vertical exposures. Therefore, the
thickness of most units is estimated and descriptions of physical characteristics such as
texture, stratification, and composition are based on observations at a small number of
localities. Particle size is expressed in terms of the modified Wentworth scale (Ingram,
1989), and sorting is expressed in the terminology of Folk and Ward (1957). Residuum
and artificial fills of limited extent were not mapped. Contacts between surficial units
may be gradational, and mapped units locally include deposits of another type. The
distribution and inter-relationships of surficial deposits are accurately depicted on the
geologic cross sections (plate 3), but thicknesses are exaggerated for diagrammatic
purposes.
Figure 7 shows the geologic time chart adopted by the Colorado Geological
Survey which follows recommendations of the Geological Survey of Canada (Okulitch,
2002), with modification of some age boundaries according to recommendations of the
International Commission on Stratigraphy (2005). The nomenclature for eras, periods,
and epochs and the corresponding age boundaries summarized in figure 7 are used in this
report. Numerical ages have not been obtained for any of the surficial units in the
Maysville quadrangle. The ages assigned to surficial units are estimates based principally
on stratigraphic relations, position in the landscape, degree of erosional modification,
differences in degree of weathering and soil development, and correlations with deposits
elsewhere in the region whose ages have been determined by numerical-dating methods.
For example, middle and early Pleistocene alluvial units are correlated with nearby
deposits in the Nathrop quadrangle that contain dated volcanic ashes (Van Alstine, 1969).
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