past Helperby and Myton-on-Swale before joining the River Ure. River levels Low and High Water Levels are an average figure. Natural history The River Swale and its valley support a range of habitats including broadleaved, mixed and conifer woodland as well as hay meadows and grasslands. Limestone scar, bracken, scrub and heather moorland can also be found. Amongst the species of tree that can be found are ash, birch, rowan and bird cherry along with shrubs such as hawthorn, hazel and holly. There are smaller populations of yew and sycamore. Pine, larch and spruce occur mostly in plantations with alder and willow common along the river banks. The many hay meadows are filled with buttercup species and wood cranesbill. Geology There are three distinct geological areas in Upper and Mid Swaledale. The upper reaches of the river flow over Carboniferous, Permian and Triassic rock, all of which are atop a layer of Lower Paleozoic beds. These rocks are rich in minerals and metalliferous sulfide ores such as galena, sphalerite, chalcopyrite and bravoite. There are also deposits of fluorite, barite, witherite, calcite, dolomite and barytocalcite. Along the valley sides can be seen the typical Dales geology of Yoredale beds, alternating strata of limestone and gritstone. Small seams of coal, particularly around Tan Hill, have been found and worked. During the last Ice Age, the glacier that dominated the valley was responsible for broadening it and altering the course of the river around Keld and Round Howe. It was also responsible for cutting the Kisdon gorge. Retreat moraines lower in the river valley can be seen around Gunnerside and Grinton Bridge. History The earliest evidence of occupation in the river valley can be dated to the Mesolithic and Neolithic Ages with the discovery of flint tools and arrowheads. Around Harkerside are some small stone circles that date to the Bronze Age and some Iron Age defensive earthworks. Evidence of lead mining has been traced back to Roman times with finds at the Hurst mine. This industry seemed to decline until after the Norse (Danish) invasions of the area. During the major ecclesiastical building of the 12th and 13th centuries, lead became a valuable commodity and mining once again increased in the valley. Evidence of the lead mining can still be seen from the remains of the 18th-century practice of 'hushing' that involved creating turf dams across gills that were then released to