ORIGIN AND DEVELOPMENT OF RILL MARKS ON THE BEACH FACE AT GALVESTON ISLAND STATE PARK, TEXAS

In Galveston Island State Park, Texas, well developed rill marks are present on the beach face. These rill marks are produced as the tide goes out and the water drains from the berm and bars that are found on the beach face. They develop in zones where there is a higher permeability in the berm and bars allowing water to drain from the beach face. These zones contain a higher concentration of course clasts and small amounts of clay. Below and above each permeable zone, the sand contains fewer course clasts and a larger clay percent resulting in these zones being less permeable. Rill marks develop by water that flows out of the sand and down the beach face cutting a channel perpendicular to the beach following the breaking of each wave as the tide goes out. Rill marks are divided into five different types: fringing, conical, branching, meandering, and bifurcating. All five types of rill marks are found on the beach face at Galveston Island State Park. The type of rill mark that developed is determined by: the angle of the slope on the berm and bar face, the amount of porosity and permeability of the sand, and wave impact direction and intensity. Rill marks are best developed on stormy days with high tides. Preservation of rill marks in the rock record is low, because the next high tide destroys the rill marks that developed during the last low tide.