Exotic pebbles are present in trace amounts (less than 1%) in most gravel deposits (fig. 7). The exotics include, in relative order of abundance, quartzite, quartzose sandstone, dark flint, and a single piece of weathered granite. Some petrified wood may also be exotic. Most exotics are pebbles, but a very few are cobbles, up to 9 cm long. They are usually well rounded and polished. Quartzite pebbles display typical metamorphic features: schistose or gneissic fabrics, undulatory extinction of quartz grains, corroded or sutured grain boundaries, and veins of biotite or epidote. These pebbles fall into three general color groups: about 60% are yellow, orange, and light brown (5Y, 5YR, 10YR); about 30% are pink, red, and red-purple (5R, 10R, 5RP); and 10% are purple, dark brown, and gray (5P, 5Y, 5YR, N) (Aber, 1985).
Fig. 7. Selection of exotic pebbles from upland chert gravel in the lower Walnut basin in southern Butler County. Pebbles are quartzite and quartzose sandstone. Swiss knife for scale.
The lithology of exotics closely matches that of the arkosic alluvium of the Wellington-McPherson-Arkansas River Lowlands and the basal Cretaceous conglomerate of the Smoky Hills region (Aber, 1985). Exotic pebbles are trace constituents of upland gravel deposits in most portions of the study region. However, exotics have not been discovered along the Marais des Cygnes valley in southern Osage and northeastern Lyon counties. Nor have exotics been found along the South Fork of the Cottonwood River valley in central Chase County. Exotic pebbles are seemingly most abundant in gravels of the lower Walnut basin and on the Missouri-Arkansas divide in Anderson County. In many cases, exotic pebbles tend to be locally more abundant in higher (that is, older) gravel deposits.
Kansas Geological Survey
Web version March 18, 1998
http://www.kgs.ku.edu/Current/1997/aber/aber7.html
email:lbrosius@kgs.ku.edu