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High Force is a waterfall on the River Tees, near Middleton-in-Teesdale, Tees Valley, England High Force was formed where the River Tees crosses the Whin Sill - the rock system followed by Hadrian's Wall. The waterfall itself consists of two different types of rock. The upper band is made up of whinstone, a hard rock which the waterfall takes a lot of time to erode. The lower section is made up of carboniferous limestone, a softer rock which is more easily worn away by the waterfall. The wearing away of rock means that the waterfall is slowly moving upstream, leaving a narrow, deep gorge in front of it. The length of the gorge is currently about 700 metres. The bedload (rocks that the river is carrying) is mainly composed of large boulders, which are rolled along the river bed. Upstream of the waterfall, the river is narrow; downstream, it widens and meanders.