 As the Cotswold dip-slope is followed toward the south-eastern county extremities these clays and mudstones begin to predominate as the Upper Jurassic period Oxford Clay.Most of the Cotswold rivers flow south-eastwards down the dip-slope, supplying the river Thames, whose headwaters lie at the bottom of the slope on the Upper Jurassic clays.Terrace deposits up to 6m thick of mainly local oolitic limestone gravels have been left by the river Thames in its upper course during glacial and peri-glacial periods.