The major purpose of this study was to examine in detail the anomalous transgressive "boundary" sandstones which generally are present at formation boundaries throughout the Parras and La Popa basins. These extensive sheet-like sandstone bodies are massive, resistant units up to 20 m thick and up to 25 km wide that form the cap of mountains and hogbacks. These are anomalous units because transgressive deposits thicker than 5-10 m are rare in ancient deposits although they are well-documented for some Holocene deposits. For example there are no known thick transgressive deposits in the well-studied cyclic transgressive-regressive sequences in the Cretaceous rocks of the Rocky Mountains. McBride and others (1975) previously interpreted these boundary sandstone bodies to have been laid down as thick transgressive or regressive blanket deposits as the shoreline moved back and forth across the basins in response to variable basin subsidence and sediment input. In order to clarify this interpretation, we selected deposits formed during the latest part of the first deltaic progradation represented by the Cerro Huerta Formation, and at the beginning of the first marine transgression represented by the Cañon del Tule Formation for detailed study. / text