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An organic rich, gleyed soil (Cumulic Haplaquoll) on the edge of a sag pond near San Marino High School illustrates the effects of poor drainage and tectonism on soil formation (pedogenesis) in a semiarid region during the Holocene. Combined with other characteristics of soil development, the organic matter masks the presence of six depositional strata in the granodioritic alluvium in which the soil formed. Two of these units occur only on the south side of a strand of the Raymond fault, three occur only on the north side, and one occurs on both sides.

The dominant clay minerals are smectites that have formed from soil solution within the past 10,000 years. Beidellite formed in horizons with exchangeable aluminum (pH less than 6.5), and montmorillonite formed in horizons without exchangeable aluminum (pH greater than 6.5). A soil tongue produced by fault movement was relatively unweathered after it was emplaced by an earthquake that occurred less than 1,400 years ago. Compared to other Holocene soils, pedogenesis at this site is intermediate between the weakly developed Hanford soils formed in granitic alluvium in the San Joaquin Valley and the strongly developed Concepcion soils formed in continental terrace deposits along the coast.

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