The Triassic is represented by the red mudstones and sandstones of the Sherwood Sandstone Group that underlie much of the lower lying Vale of York and the valley of the River Swale. However, throughout much of the area they are covered by glacial and post glacial deposits. The Triassic rocks of Britain were deposited under arid conditions over a large desert plain with numerous basins and mountain ranges. Lower Triassic sandstones represent the deposits of large braided rivers that crossed the desert plain. These are overlain by the Mercia Mudstone Group, which comprise mudstones, sandstones and thin horizons of gypsum and rock salt deposited over the desert plain in large, shallow basins. The mudstones of the overlying Penarth Group represent a transition to increasingly marine conditions.
Photo: River Leven – North bank of the River Leven near Sutterskelfe Hall, Stokesley, showing alluvial silts and clays overlying sand and gravel which rest on disturbed and contorted Triassic mudstones and siltstones of the Mercia Mudstone Group.