The rocks that crop out in Lyon County are of sedimentary
origin; their areal extent is shown on Plate 1. The rocks
exposed at the surface range in age from Pennsylvanian to
Recent. In pages devoted to descriptions of the rock succession
of Lyon County, statements of thickness and distribution apply
to this county only, unless otherwise indicated. Recent and
Wisconsinan Stages Deposits of stream-laid gravel, sand, silt,
and clay, together with occasional cobbles and boulders, occur
in the principal stream valleys and constitute the floodplains
of the valley. The maximum accumulations are 40 to 50 feet
thick. Deposits in the smaller valleys are thinner and more
variable in lithology. The coarse fraction of the alluvium of
Cottonwood River valley contains considerable amounts of well-
rounded quartz sand derived from Cretaceous sandstones in the
headwaters area of the river. Marais des Cygnes and Verdigris
rivers and Neosho River above its junction with Cottonwood River
contain only small amounts of rounded quar