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. Downcutting and a gradual southerly migration of the river has left a succession of four terraces at varying heights, chiefly north of its present course.