By comparing USDA soil-survey maps of varying ages, one can discern relationships that are not otherwise apparent. Maps of a single county from 1909, 1938 and 1980 illustrate that point. Early soil maps were largely a reflection of a region’s geology. Modern soil maps, while still retaining information about geology, are based on Soil Taxonomy. They contain greater information about the multitude of properties within a soil. That classification system is useful because it groups and organizes soil properties based on (1) soil horizons and (2) long-term soil temperature and moisture.