H. B. Willman and John C. Frye
The Pearl Formation is named for Pearl, Pike County.
The type section is an exposure in a box canyon 1 mile southwest of Pearl, SE SW NE Sec. 16, T. 7 S., R. 2 W., Pike County (Frye and Willman, 1965a, p. 14, Pearl Prairie Section, units 1-4). In the type locality the Pearl Formation is a deposit along the front of the Mendon Moraine where the Illinoian glacier mounted the west bluff of the Illinois Valley and blocked drainage from the upland to the west. It is largely a pebbly sand about 40 feet thick with beds dipping steeply southwest, and it appears to be an ice-front delta.
The Pearl Formation is Illinoian outwash, but it may include Kansan outwash in some deep valleys. The formation is restricted to the outwash that overlies or extends beyond Illinoian till. Sand and gravel in the till and intratill members of the Glasford Formation may be continuous into the Pearl Formation, but in nomenclature they are separated from it by a vertical cutoff (fig. 7). Deposits in the same stratigraphic position that are dominantly silt and clay are assigned to the Teneriffe Silt. The Pearl and Teneriffe are never superimposed and are separated by vertical cutoff. Intertonguing and gradational units are described in informal facies classification.
The Pearl Formation has essentially the same lithologic variations as the Wisconsinan Henry Formation, but the subdivisions are not widely enough distributed to merit classification as members. The deposits are generally more oxidized than those of the Henry Formation, but their differentiation is based largely on the presence of the Sangamon Soil in the top of the Pearl, or, when the Sangamon Soil is missing, the presence of the Roxana Silt above the Pearl.
Extent and thickness
It is about 40 feet thick. The Pearl Formation most commonly occurs in terraces along valleys near the margin of Illinoian glaciation, except in the major valleys where the surface of Illinoian aggradation was lower than the Wisconsinan and the Illinoian outwash is buried or eroded. The Pearl Formation is also present as outwash on the Illinoian till plain, mainly in front of Illinoian moraines and in isolated kames and crevasse deposits. In the complex relations of the Kaskaskia ridged drift, the outwash is not readily differentiated and is included along with the youngest till in the Hagarstown Member of the Glasford Formation.
It consists of sand and gravel that has the Sangamon Soil in its top. It overlies Illinoian or older drift or bedrock.
Illinoian outwash sands and gravels assigned to the Pearl Formation are described by Frye et al. (1969, Mt. Carroll South Section); Frye and Willman (1965a, Marcelline and Lost Prairie Sections); and Shaffer (1956, Hazelhurst and Mt. Carroll Sections).
FRYE, J. C., H. D. GLASS, J. P. KEMPTON, and H. B. WILLMAN, 1969, Glacial tills of northwestern Illinois: Illinois State Geological Survey Circular 437, 47 p.
FRYE, J. C., and H. B. WILLMAN, 1965a, [Illinois part of] Guidebook for field conference C—Upper Mississippi Valley (R. P. Goldthwait [organizer]; C. B. Schultz and H. T. U. Smith [eds.]): International Association of Quaternary Research 7th Congress, Nebraska Academy of Science, p. 81-110; Illinois State Geological Survey Reprint 1966-B (supplemental data, J. P. Kempton and H. D. Glass, p. C-S1-C-S11), 41 p.
SHAFFER, P. R., 1956, Farmdale drift in northwestern Illinois: Illinois State Geological Survey Report of Investigations 198, 25 p.
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