The 1973 Soil Survey of San Diego County revealed a distintive soil series in the mountainous regions east of San Diego. Pedologists involved in the Soil Survey named it the La Pasta Series. The La Pasta series is characterized by the following horizons: R (unweathered granodiorite), Cr (weathered granodiorite), C (medium to coarse-grained loamy sand), A (medium to coarsed loamy sand with high organic content). Previous work by students at San Diego State University showed that the Cr- and C-horizons developed from the R horizon primarily as the result of plagioclase, and to a lesser extent K-feldspar, biotite, and hornblende weathering. However, the A horizon was poorly exposed at the initial study site and was not sampled. In order to assess how representative the previous work is of the mollisols that characterize the La Pasta Series, an additional profile was sampled. The new profile was located ~30-40 m west of the earlier profile, and includes a well developed A horizon. New geochemical and petrological analyses indicate that the La Pasta Series mollisols are composite, consisting of a paleosol (the R, Cr, and C horizons) and an overlying significantly younger soil (the A horizon) that formed under the modern day climatic regime.
