At from its source to the sea, the Arun is the longest of the rivers flowing entirely within Sussex. It rises as a series of streams, known locally as "ghylls" or "gills", to the east of Horsham, in St Leonard's Forest. It flows westwards, along the southern boundary of Horsham and turns briefly to the north to skirt Broadbridge Heath. Continuing westwards, it is joined by the North River, which is also known as the River Oke, whose headstreams are the heights of Leith Hill and Holmbury Hill in Surrey. After the junction, it passes under the A29 road, which follows the route of the Roman Stane Street at this point, and timber piles of a Roman bridge have been found in the riverbed. The earthworks from a Roman station are close by. To the south of Rudgwick it is crossed by a disused railway line, and at this point it crosses the contour. Its course is marked by winding meanders as it turns towards the south, and the county boundary briefly follows its course, the River Lox / Loxwood Stream joins the Arun at Drungewick just before it is joined by the partially restored Wey and Arun Canal. Its former course to the west of the canal can be clearly seen, and is followed by the boundary, but the main flow of the river follows a new straight cut just to the east of the canal. Once the boundary crosses back over the canal, the river resumes its meandering course on the eastern side of the canal.