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.