Volcanic ash-flow tuffs are common in the Pacific Rim countries of Latin America. In addition to being consolidated by geologic processes, they are cemented pedogenically with illuviated silica and sometimes also with carbonates. These deposits, calledtepetatein Mexico, have very low permeability and the overlying soil erodes rapidly when cultivated. This study characterized the density and strength of typical tepetate. Density varied from 1.00 to 2.11 Mg/m3, and the mean of all 18 samples was 1.38 Mg/m3. Unconfined compressive strength varied from 3 to 160 kg/cm2, and the mean was 24.7 kg/cm2. The strongest, most indurated tepetate appears to occur when disseminated carbonates combine with silica as cements.