One hundred twenty samples were collected from measured stratigraphic sections in the Delmar, Friars and the undifferentiated Delmar and Friars Formations. The samples were subjected to clay mineral analyses by x-ray diffraction and the abundances of the clay minerals were determined. The principal clay minerals are expandable clay minerals (smectitic and mostly vermiculitic), with lesser kaolinite and illite. Chlorite was found in trace amounts in some samples, predominantly in the Friars Formation. Mixed layer clays were also found in a few samples. Some samples were subjected to grain size fractionation and the relationship between mineralogy and grain size was observed. Cation-exchange capacities of samples were also determined. The mineralogic and stratigraphic variations were interpreted in terms of provenance, depositional processes, diagenesis, and climatic changes. Distribution of clay minerals in the Delmar, Friars and undifferentiated Delmar and Friars Formations is controlled by several factors, such as variations in provenance, climatic conditions of weathering differential sorting of transported particles, variations in current velocities, tidal cycle and the action of tidal currents and variations in depositional environments. No reliable evidence of post depositional alteration of clay minerals was found in x-ray diffraction patterns. Their composition is controlled largely by composition of their source areas and climatic conditions of weathering. All clay minerals but kaolinite were formed under arid to semi-arid climatic conditions. The clay minerals in the Delmar Formation were derived predominantly from diverse source rocks or minerals. The source of kaolinite is Paleocene or Early Eocene paleosol which was formed under humid climatic conditions. Different depositional conditions such as estuarine, lagoonal , tidal flat and channel environments were found in the Delmar Formation. Less diversified source rocks or minerals for the clay minerals, as well as depositional environments, were found in the Friars than in the Delmar Formation. The sediments were deposited predominantly in the estuarine environment. The undifferentiated formation generally shows more similarities to the Friars than the Delmar Formation. It shows similar characteristics to the Friars than the Delmar Formation. It shows similar characteristics to all three portions, (bottom, middle and top) of the Friars Formation. However, it generally shows the characteristics of the top portion of the Delmar Formation.