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Fragipans are a common feature in Pennsylvania, occurring in 30% of Pennsylvania' soils. These fragipans resist root penetration and are characterized by very coarse prismatic structure, firm to very firm brittle consistence, low permeability, bulk densities that are higher than overlying horizons, loamy textures, and a low organic matter content. An index of fragipan expression (weak, moderate, and strong), based on firmness, brittleness, permeability, resistance to root penetration, and strength, is proposed. Fragipans form in transported parent materials: glacial till, colluvium, loess, old alluvium, and, less commonly, in lacustrine and turbated residual materials. Fragipan formation is favored in climates that promote a leaching environment under forest vegetation. Topographic effects on fragipan formation are suggested by the relationship between fragipan expression and depth and drainage. The degree of fragipan expression follows the trend: somewhat poorly drained > moderately well drained > well drained, while the depth to the top of the fragipan follows the opposite trend. Studies suggest that a weakly developed fragipan requires 6000 yr to form, while a strong fragipan requires 18000 yr. A four‐phase fragipan formation model is proposed for Pennsylvania fragipans. In Phase 1, transported parent material dries from the surface downward, forming prisms and the prism material is packed slightly. In Phase 2, clay and amorphous aluminosilicates are added to the prisms plugging some pores and creating brittleness by forming grain‐to‐grain contacts. Maximum fragipan expression occurs during this phase. The upper part of the prism begins to degrade during Phase 3 as more illuvial clay accumulates resulting in increased expansion and contraction during wetting and drying. The prisms are physically broken up and become a part of an overlying argillic horizon. The amorphous materials that formed the grain‐to‐grain bridges are leached from the upper part of the prism. By Phase 4, these processes have completely destroyed the fragipan.

Eleven well-drained soils formed in till parent materials of varying ages in northeastern Pennsylvania were studied to determine changes in the soils with time. Four profiles (three Lackawanna and one Bath) were formed in Woodfordian till (15,000 yr B.P.), and two (Leck Kill) were formed in Altonian till (>28,000, <75,000 yr B.P.). The remaining five (Allenwood) were formed in pre-Wisconsinan till (>75,000 yr B.P.). In these soils, the extractable iron oxide, extractable aluminum oxide, and kaolinite contents increase with age, as do the total clay and fine/total clay ratio. With increasing age, the maximum accumulation of these constituents is found deeper in the profile. The extractable silicon oxide distribution is constant with depth, but it decreases in overall amount with time. Gibbsite is found only in small amounts in the A horizon of Altonian soils, but occurs throughout the profile of pre-Wisconsinan soils, although only in small amounts. In general, differences were found in these soils which separated them into three groups representing varying degrees of soil development. A regression equation was derived to predict the age of soils formed from the Altonian till based on a “clay accumulation index” value for soils of known Woodfordian and Holocene ages. The equation log Y = 1.80 + 0.992(logX) best fit the data, with an r2 value of 0.913. Using this equation, a mean age of 41,000 yr was calculated for the Altonian soils. This date was used to derive a second equation to predict ages for pre-Wisconsinan soils. The equation with the highest r2 value (0.934) was log Y = 1.81 + 0.998(logX). Dates for soils developed in the White Deer till and the Laurelton till of the pre-Wisconsinan stage were calculated to be 86,000 and 91,000 yr B.P., respectively. These dates fall within ages estimated for the Sangamon Interglaciation and thus would appear to be too young for pre-Sangamonian materials. The probable reason for the “too-young age” is that the C-horizon material of the pre-Wisconsinan soils was weathered and did not provide an accurate estimate of clay accumulation for the prediction equation.

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