Geologic Map of the Maysville Quadrangle, Chaffee County, Colorado

and decomposed to be of commercial value. Thickness at least 20 ft.
Qi Illinoian () alluvium (middle Pleistocene) – Brownish-gray boulder alluvium,
well rounded to subrounded. Composed of igneous and metamorphic rocks and some
siliceous sedimentary rocks. Soil at top is strongly developed. Mapped in the South
Arkansas River near the eastern quadrangle boundary, where it forms a terrace 20-40 ft
above the Qboo terrace, and on the western side of Walden Gulch. Equivalent to unit Qg4
of Van Alstine (1969) and correlated to the Illinoian glacial stage of the midwestern
USA. Thickness at least 20-40 ft.
Qgo Older gravel deposits, undivided (early to middle Pleistocene) – Yellowish
gray to yellowish-orange, fairly well-stratified alluvium containing well-rounded to
subrounded fragments of igneous and metamorphic rocks. Mapped along Sand Creek and
Squaw Creek. Forms dissected terrace-like surfaces underlain by bouldery gravel 60-120
ft above stream level, downstream from pre-Bull Lake till. Probably composed mostly of
pre-Bull Lake outwash, but may also contain older till and local younger alluvial
deposits. Thickness probably more than 66 ft.
Qk Kansan () alluvium, undivided (middle Pleistocene) – Yellowish-gray to
yellowish-orange, fairly well-stratified alluvium containing well-rounded to subrounded
fragments of igneous and metamorphic rocks. Mapped along Blank Gulch, Walden
Gulch, and on valleys walls of the South Arkansas River. North of Buena Vista, contains
type-O Pearlette ash, a marker volcanic bed with origin in Yellowstone National Park
(Lava Creek B tuff, age 620 ka). Soil at top is strongly developed. Forms a terrace about
220-250 ft above stream level and small pediment surfaces graded to that level on the
range-front piedmont. Equivalent to unit Qg3 of Van Alstine (1969) and correlated with
the Kansan glacial stage of the Midwestern USA. Thickness probably more than 20 ft.
Qk3 Kansan () alluvium, younger (middle Pleistocene) – Yellowish-gray to
yellowish-orange, fairly well-stratified alluvium containing well-rounded to subrounded
fragments of igneous and metamorphic rocks. Mapped only in the South Arkansas River
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