Although Mesolithic flints have been found in neighbouring parishes, none have yet come to light in Horsley. Mesolithic hunters and gatherers probably passed through the area and their remains are still to be found. The oldest find in the parish is a Neolithic cup marked stone from Nafferton.
On both Horsley Hill and in Horsley Wood there are Iron Age hillforts. The Horsley Wood example lies near a valley, a position paralleled by another Northumbrian hillfort at Warden Hill.
The Roman period remains are dominated by Hadrian's Wall which runs across the north of the parish. The remains include the Wall, vallum, milecastles and turrets. Most of this section of the frontier survives as cropmarks, although an earthwork platform outlines the remains of Milecastle 15. As well as the military remains, there are native farmsteads at North Dunslawholm and near Horsley. Land boundaries may also have been created at this time and can be traced on aerial photographs and in old documents at Horsley Wood.
After the end of the Roman period, little is known about Horsley until medieval times. Villages and hamlets grew up at Welton, Whittle and Nafferton. The remains of Welton village survive as an extensive area of earthworks with building platforms along a street and ridge and furrow behind. At the east end of the village is Welton tower house built in the 14th or 15th century on the side of a manor house. There was also a hospital in the parish called the Hospital of Saint Michael as well as the Spital Well and Nafferton Castle. The castle was a potential rival to Prudhoe Castle and built without the planning permission of its day. With no licence to erect battlements, orders were sent from London over a period of time to dismantle the castle and remove its materials to Bamburgh Castle. The castle and later tower house are immortalised in the legend of Lang Lonkin, a notorious pirate who murdered the lady and her child of nearby Welton Hall, told in a well known border ballad. Much of the land in this part of the county was part of the Umfraville barony based at Prudhoe Castle. As was the right of the lord of the manor, gallows were erected at Gallow Hill overlooking much of the lower parish lands around Ovingham and Prudhoe, a prominent position designed to deter wrongdoers.
In post-medieval times Horsley was a favoured stop on the road from Newcastle to Hexham and Carlisle. Several inns catered for the travelling public, including The Crown and Anchor and The Lion and The Lamb. One 18th century visitor to the parish was John Wesley.
Horsley has been an important place for the supply of water to much of Newcastle and Gateshead since the 19th century. A series of reservoirs at Whittle Dene, just to the north of the parish boundary were built for both domestic and hydraulic-power (industrial) usage. These reservoirs involved the like of Lord Armstrong and John Dobson in their construction from 1848 onwards. Building such as the Superintendent's House still stand as does the Sulphur House. Water was carried to Tyneside through aqueducts and tunnels and further systems were tacked onto the whole of the Whittle Dene scheme as demand and supply increased, eventually extending as far as Catcleugh reservoir near the Scottish Border.
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