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(Willman and Frye 1970). Oxidation and fracturing, with iron staining on the fracture faces, typically extends 10 to 20 feet or more into the Glasford till. Compared to overlying loess deposits, the Glasford till is considerably more pebbly and dense, has a lower moisture content (11–16%), and has greater unconfined compressive strength (Qu, table 1). The upper 6 to 12 feet of Glasford Formation, where uneroded, is generally more weathered, is leached of carbonates, has a higher water content, and is less stiff than the majority of the unit.
Relatively unaltered portions of Glasford till in this quadrangle typically have a composition of about 35–55% illite in the clay mineral fraction and a loam to silt loam texture with about 19–25% <2 mm clay, 45–50% silt, and 25–35% sand (table 1). The Glasford Formation, deposited during the Illinois Episode, may also include sand and gravel lenses deposited from glacial meltwater streams within, in front of, or below glacial ice. Sand and gravel lenses are relatively uncommon on the highest areas of the bedrock-controlled uplands, but become more abundant within adjacent lowlands, terraces, and glacial hills and knolls. Similarly, pre-Illinois Episode deposits are likely not present at elevations above ~390 feet asl (above sea level) due to more limited deposition and/or postdepositional erosion.
Glacial Hills and Knolls
A few hills and knolls (~1% of map area) are found on the edge of the bedrock-controlled uplands that likely contain lithologically complex unconsolidated deposits (typical of such hills). These areas, interpreted principally to be of glacial origin rather than bedrock highs, were mapped as the Hagarstown Member of the Pearl Formation (Willman and Frye 1970, Killey and Lineback 1983). Since most of these areas are blanketed by 5 to 13 feet of loess, stipples on the map indicate the Hagarstown unit in the subsurface. One area of near-surface Hagarstown Member is mapped in the northwest portion of the quadrangle (solid reddish brown color) where the loess cover was eroded on a hillside to less than 5 feet thick. Previous studies in south-central Illinois have noted significant sand and gravel deposits in similar glacial ridges (Jacobs and Lineback 1969, Grimley 2008); however, some ridges contain a high proportion of intermixed diamicton and fine-grained sediment (Phillips 2004, Grimley 2008).
The hills and knolls regionally appear to have an association with the transitions to bedrock topographic highs, but their exact origin is not yet clear. Sediment within the hills is interpreted as ice-contact and may include various genetic types such as debris flows, melt-out till, and ice-marginal channels deposits. Although few new observations of Hagarstown deposits were acquired in this quadrangle, one water-well log from a typical hill (27139; Sec. 11, T2S, R7W; cross section A–A9) notes the presence of about 15 feet of loose fine sand, along with beds of clay and sand and zones with clayey diamicton, perhaps debris flow deposits. In Dutch Hill (SW¼, SW¼, NE¼, Sec. 10, T3S, R7W), an unpublished ISGS report by J.W. Baxter and N.C. Hester noted 14 feet of loess over 3 feet of sandy, pebbly silt over 16 feet of fine to coarse sand with few, small pebbles (interpreted as Hagarstown Member) from a power-auger boring, suggesting a kamic origin. In other similar hills to the north, variable materials have been observed, such as well-sorted to poorly sorted sand interbedded with loam, diamicton, and inclusions of pre-Illinoian sediments (Grimley 2008). The upper 3 to 10 feet of the Hagarstown Member, below the loess, is typically altered to a clay loam to sandy clay loam and contains pedogenic alteration features, such as clay skins and root traces that formed during interglacial soil development (Sangamon Geosol).
Broad Terraces and Tributary Valleys
Areas of broad terraces and tributary valleys (together ~42% of map area) are found in much of the northeastern and southwestern portions of the quadrangle along and adjacent to Mud Creek, Little Mud Creek, and Doza Creek valleys. These areas, which tend to overlie former topographic lows on the bedrock surface (fig. 2), have been periodically infilled with lacustrine and fluvial deposits. Deposits are mainly fine-grained and stratified, but include some coarse-grained materials. The various terrace levels were formed as a result of alternating periods of sediment aggradation (mainly during glacial times) and river incision (mainly during interglacial times). The terraces observed today were formed as a result of processes during the last two glaciations (Illinois and Wisconsin Episodes) as well as during interglacial and postglacial times. Approximately 7 to 13 feet of loess (Peoria and Roxana Silts) covering the Illinois Episode terraces in uneroded areas distinguishes these from the younger terraces. In places, older pre-Illinois Episode slackwater deposits are preserved in the subsurface. It is conceivable that some high terraces visible today are, in part, palimpsest surfaces, with the younger deposits draped or superimposed on pre-Illinois Episode deposits and their former terraces.
Two divisions within a loess-covered Illinois Episode terrace are mapped: (1) loess-covered stratified sand and gravel (Pearl Formation) and (2) loess-covered accretionary or stratified fine-grained deposits (Berry Clay Member or Teneriffe Silt). All such areas are mapped as loess and given an appropriate diagonal line pattern and colored to indicate where more than 5 feet of either the Pearl Formation (reddish orange) or the Berry Clay Member-Teneriffe Silt (brownish gray), respectively, are predicted to occur at depth. In most cases, areas mapped as having Pearl Formation also have a thin overlying deposit of Berry Clay Member, but such areas are mapped as Pearl Formation because of the practical importance of the coarser-grained deposits.
The Pearl Formation terrace is mapped principally in the western portion of the map, south of New Athens and at elevations from about 420 to 440 feet asl. The Illinois Episode
