In places this impervious
dolerite rock, with shallow soil above it, prevented the growth of scrub or trees: this enabled certain post-glacial Arctic / Alpine plants to survive here when elsewhere as a rule they were overgrown. The
Sugar Limestone formed by thermal
metamorphism of the limestone into which the Whin Sill was intruded also meets the requirements of some of these plants. Teesdale is famous among naturalists for the "Teesdale Assemblage" of plants found together here that occur widely separated in other locations, abroad or in the
British Isles. Part of
Upper Teesdale near the
Cow Green Reservoir is designated a
National Nature Reserve; it contains the unique Teesdale Violet and the blue
Spring Gentian as well as more common Pennine flowers such as
rockrose, spring sandwort, mountain pansy, bird's-eye primrose and
butterwort. Hay meadows in the valley above High Force, some now carefully cultivated to ensure this, contain an extremely rich variety of flowering plants including globe flower, wood cranesbill and Early Purple Orchid. On the south bank of the Tees near High Force can be seen the largest surviving juniper wood in England. ==Geography==