Glacial outwash sand and gravel from three northern Kentucky counties in the Ohio River Valley have been analyzed in order to characterize their particle size, composition, morphology, and surface alteration and to determine the geologic significance of lateral and stratigraphic variations in these sediment properties. The late Wisconsinan-age deposits, actively mined in terraces along the Ohio River, are composed of moderately sorted sand and a poorly sorted, bimodal mixture of sand and gravel. A systematic pattern of regional grain-size variation was found to be associated with the confluence of the Great Miami River, a major meltwater sluiceway entering the Ohio River Valley. This area contained the greatest amounts of large-pebble gravel. Downstream from this area, the maximum and modal gravel size and the total gravel percentage decreases, the total sand percentage increases, but essentially no change takes place in modal sand size. Sediment sorting is generally poor, with no apparent downstream variation. Over these broad, regional trends are superimposed local textural variations that correlate with hydraulic gradients created by meander bend geometry. Stratigraphically older outwash located in adjacent upland areas is much coarser grained and more poorly sorted than outwash in the Ohio River Valley.
Outwash particle composition is related to grain size, stratigraphy, and location. Approximately 75 percent of the gravel to coarse sand sizes are composed of dolomite and limestone, with the remaining 25 percent made up of quartz, chert, sandstone, igneous, and metamorphic rock fragments. Rock fragment percentages, in general, decrease from coarse pebble to fine sand sizes, as individual mineral percentages increase in the sand fractions. Chert and quartz sandstone are most abundant in the fine pebble sizes.
Stratigraphic differences in sand and gravel composition, reported for ice-laid tills of the region, were used to differentiate between subsurface samples of outwash in the Ohio River Valley. Limestone/ dolomite ratios greater than 5:1 and an abundance of chert and sandstone characterize older outwash units, while a limestone/ dolomite ratio less than 1:1 typifies the youngest unit. Regional composition changes along the Ohio River Valley include a downstream decrease in carbonate rock pebbles and an increase in chert, sandstone, igneous, and metamorphic rock pebbles. Local variations in pebble composition are found in low-energy depositional environments where clay, chert, and coal pebbles accumulated.
Pebble shape varies little with respect to particle size or stratigraphy, but it is distinctive in certain rock types. Axial ratio plots show very platy and elongated pebbles are composed mostly of limestone and sandstone. On average, outwash pebbles are rounded to well rounded, but significant variations in roundness occur with respect to lithology, grain size, and stratigraphy. Roundness variations among lithologies reflect differences in their hardness or resistance to abrasion. Having traveled a greater distance from their glacial source, pebbles in the youngest outwash are more rounded than pebbles in older stratigraphic units. Because of their longer exposure to abrasion, smaller pebbles are more rounded than larger ones.
Pre-Illinoian and Illinoian Pleistocene outwash, located on uplands bordering the Ohio River Valley, are unfavorable as a source of aggregate because of the abundance of deeply weathered, unsound particles and the large amounts of deleterious sandstone and chert. Younger Wisconsinan outwash deposits within the Ohio River Valley, in contrast, contain the largest concentrations of sound, coarse and fine aggregate with the least amount of deleterious materials.
Report of Investigations 8
Digital Object Identifier (DOI)
Amaral, Eugene J., "Sand and Gravel Resources Along the Ohio River Valley in Boone, Gallatin, and Carroll Counties, Kentucky" (1994). Kentucky Geological Survey Report of Investigations. 48.