The diachronous Upper Lias Sands of southern England are comprised of small scale rhythmic alternations of friable, yellow sandstone and firm, gray, calcareous sandstones. In general, the alternating layers are parallel to bedding and to one another; exceptions occur where the calcareous sandstones mark out the bases of erosional structures. Individual carbonate-rich or deficient horizons cannot be correlated between outcroups 2-3 miles apart, but within a single outcrop they preserve a remarkable uniformity in vertical sequence. Petrographic studies demonstrate that original textural relationships are retained in the calcareous sandstones, while the friable sandstone interbeds have undergone compaction effects. Field evidence indicates that the calcareous sandstones were lithified before the deposition of more than 6 inches of superimposed sediment. Such lithification is estimated to have taken approximately 150 years. The alternations are considered to be the result of fluctuating rates of detrital deposition: the calcareous sandstones representing temporally, and spatially restricted locales characterized by little or no detrital deposition, and the sandstone interbeds representing contiguous areas of more rapid detrital deposition. Within the Upper Lias Sands a gradual increase is observed in the relative proportions of CaCO 3 to detritals. This is believed to reflect the southerly lateral migration of the wave of maximum terriginous sedimentation. This process established conditions suitable for deposition of the succeeding limestone sequences an environment foreshadowed by the calcareous sandstones of the Upper Lias Sands.