Early examples include prehistoric walls and tombs such as Skara Brae and Maes Howe in the Orkney Islands off the Northern Coast of Scotland. Maes Howe dates back to around 3,000 BC. In British agriculture Dry Stone walling has been used since the early Middle Ages for buildings and animal pens. Most existing walls are the result of the extensive enclosure of farmland in the 18th and 19th centuries, when the first professional wallers appeared. This transformed the landscape which previously consisted of vast tracts of open land, common grazing land, heath and moorland. Not only are there various different styles of stone wall around the UK but different stone is used depending on what can be found naturally in the area. The Cotswolds dry stone walling usually comprises of Oolitic limestone which is relatively soft and easily worked - forming walls which are tightly packed and neatly finished with smaller stones. Cornwall sees the heavy use of slate and walls are often constructed in a herringbone pattern. The infill between the slate is usually earth which accounts for the flowers and grasses which can often be seen growing from Cornish walls. Derbyshire walls tend to be more irregular due to the course sandstone which is difficult to work into even blocks. Likewise, North Yorkshire stone walls are often irregular whilst South Yorkshire often produces a more even and compact bond.