Stane Street (Chichester) - Surveying and Construction

Surveying and Construction

The direct line from London Bridge to Chichester passes over the North Downs at Ranmore (200 m above Ordnance Datum), the Greensand Ridge at Holmbury St Mary (260 m OD) and over the South Downs near Goodwood Racecourse (167 m OD). The steep gradients which would have been required if the road had followed a direct line would not have been practical for wheeled traffic and so the Roman engineers designed the road to cross the North Downs by natural breach cut by the River Mole and to pass to the east of the high ground of Leith Hill. The geology of the region was also considered and the road leaves the direct line at Ewell to move onto the well drained chalk of the North Downs, in preference to remaining on the London Clay. The road is able to make a more gentle ascent of the South Downs escarpment at Bignor than was possible at Goodwood and the chosen route avoids the need for the road to cross the steep sided River Lavant valley at East Dean.

In order to accommodate and exploit the complex topology of the region, the road used four main straight sections (sometimes referred to as limbs) connected by shorter linking sections. Each limb could be surveyed separately using local vantage points. The major limbs were:

• London Bridge to Ewell
• Ewell to Mickleham Downs
• South Holmwood to North Heath near Pulborough
• Pulborough to Chichester

The limbs were not joined directly to each other, but are linked instead by shorter segments.

The Roman surveying technique is clearly demonstrated by the longest of the four limbs from South Holmwood to North Heath (24.5 km or 15 miles). The line that the road follows runs between two prominent hill tops, Borough Hill on the South Downs and Brockham Warren on the North Downs. There are some short local variations from the straight line of the limb, for example the Roman road loops to the west for around 200 m at Okewood Hill to cross a small stream at a convenient fording point. South of North Heath the road turns by an angle of 7 degrees to head towards the crossing of the River Arun at Pulborough. North of South Holmwood, the road turns by a further 7 degrees to the north to approach Dorking.

The average width of the paved road is 7.4 metres (24 ft), or 25 Roman pedes. This is wider than the average 6.51 metres (21.4 ft) or 22 pedes for Roman roads in Britain. The overall width between the outer ditches, which can still be seen on aerial photographs taken over the South Downs, is 25.6 metres (84 ft) or 86 pedes. The actual width of metalling varies from place to place, and the outer ditches were found to be 27.4 metres (90 ft) apart at Westhampnet. Sections of intact road that have been excavated in several places show a variety of local materials, with the agger often being constructed of alternating layers of sand and gravel paved with large flint nodules, or sandstone, surfaced with smaller flint or sand and gravel. The metalling was generally about 30 centimetres (1 ft) thick at the centre with a pronounced camber. Near to the Alfoldean station the metalling was constructed from iron slag in a solid 30 cm thick mass.