At the end of the last Ice Age the Bristol Channel was dry land, but later the sea level rose, particularly between 1220 and 900 BC and between 800 and 470 BCE, resulting in major coastal changes. The Somerset Levels became flooded, but the dry points such as Glastonbury and Brent Knoll have a long history of settlement, and are known to have been occupied by Mesolithic hunters. The county has prehistoric burial mounds (such as Stoney Littleton Long Barrow), stone rows (such as the circles at Stanton Drew and Priddy) and settlement sites. Evidence of Mesolithic occupation has come both from the upland areas, such as in Mendip caves, and from the low land areas such as the Somerset Levels. Dry points in the latter such as Glastonbury Tor and Brent Knoll, have a long history of settlement with wooden trackways between them. There were also "lake villages" in the marsh such as those at Glastonbury Lake Village and Meare. One of the oldest dated human road work in Britain is the Sweet Track, constructed across the Somerset Levels with wooden planks in the 39th century BC, partially on the route of the even earlier Post Track.