This Supplement to the Desert Project Guidebook presents new study areas 23-27, and additional information for other study areas and thin section studies. New study area 23 is near pipline and power line roads that cross terraces and soils ranging in age from late Holocene to late middle Pleistocene. In area 23, morphology of the laminar, plugged, and adjacent horizons of an Argic Petrocalcid was related to the radiocarbon chronology of their carbonate. This pedon was the first so studied in the Desert Project and the first to be reported in the world literature on genesis of carbonate horizons.

Study area 24 illustrates a complex of Typic Haplocalcids and Argic Petrocalcids on a dissected landscape with ridge remnants of late middle Pleistocene age and the transition to a broad stable landscape of the same age dominated by Argie Petrocalcids. Area 24 illustrates a facies change from low-gravel to high-gravel materials and profound changes in carbonate morphology (stage III to IV) and soil classification (Typic Haplocalcid to Typic Petrocalcid) that accompany the facies change. Thin sections show evidence of dissolution of primary grains in the calcic horizon of the Haplocalcid.

Study area 25, in lce Canyon of the Organ Mountains, illustrates a bedrock-deserted ancient fan,and an Argie Ustic Petrocalcid dating from middle to early Pleistocene. This soil has a red, clay Bt horizon with a subhorizon that has more than 70% clay,and illustrates stage IV of carbonate accumulation in soils of the mountain canyons. Thin sections show argillans on sand grains but not on ped faces. Prominent striae of oriented clay occur within peds and may represent former argillans.

Study area 26 is along and near the scarp of a middle Pleistocene relict basin floor that borders the Rio Grande valley. At area 26, the deep petrocalcic horizon of a Typic Petroargid illustrates stage IV of carbonate accumulation in low-gravel materials. Thin sections show evidence of disso­lution or primary grains in the petrocalcic horizon and accumulation of silica below it. Electron microscopy, soil column and culture studies indiate that soil microorganisms are involved in precipitation of fine-grain calcite in horizons of carbonate accmulation. Microscopic and chemical evidence indicates that palygorskite was neoformed in the petrocalcic horizon.

Study 27 is in a distinctive scarplet terrain in which the scarplets cut fan•piedmont sediments derived largely from sedimentary rocks such as limestone.  The scarplets expose a Holocene Haplocalcid and the  Bt horizon of an  underlying  Calciargid of late Pleistocene age.  An argillic.horizon has not formed in high-carbonate parent materials or middle Holocene age, but has formed in the underlylng soil of late Pleistocene age. This suggests that moister climates of Pleistocene pluvials may have been involved in leaching the bulk of the carbonates so that the argillic horizon could form. Thin sections of the buried argillic horizon show both prominent argillans and some limestone grains. This shows that not all of the primary carbonate must be leached from the parent materials for an argillic horizon to form. Argillans were not found on limestone grains.

Photomicrographs illustrate illuvial clay and carbonate in soils that range in age from late Halocene to middle to early Pleistocene. Coatings of oriented clay on sand grains and pebnbles (grains argillans) are chracteristic of the Bt horizons. Grains argillans of many Bt horizons have been partly to completely obliterated by cabonate. Calcified root hairs, calcite filaments and framework grain coatings (calcitans) are the youngest forms of carbonate accumulation. They are currently forming and are the major morphological expression of carbonate in late Halocence soils. Progressively older carbonate forms increase in density and hardness with increasing carbonate content. Thin sections show evidence of dissolution of primary grains in soils of late-middle Pleistocene age and older.