Lennox Bridge spans the deep gully of Lapstone Creek at the head of the steepest part of Mitchell's Pass. The foundations are on bedrock, with the water running through a channel cut by Lennox's convicts in the bed of the creek. The bridge is quite small, with a span of only 20 feet (6 metres). The single arch is built of ashlared stone blocks mortared together. The arch on the south side has a keystone bearing the inscription 'DAVID LENNOX', on the north side, 'AD 1833', the packing of the bridge to either hand of the main ashlared section is of coursed rubble revetments, making the total length of the bridge carriageway of almost 47 feet (15 metres). The stone parapets are laid above a projecting plinth which begins just above the keystones. The area between the curve of the arch and the horizontal carriageway was packed with small rubble stone. There was originally a damp-proof lime-mortar course above the vaulting, but this, together with most of the packing, was removed when the bridge was stripped back to the arch in 1976. During these repairs new concrete supports were inserted to bear the weight of a new carriageway utilising steel girders to take the traffic then still using Mitchell's Pass. These changes arc not visible, but the insertion of new blocks of stone and repairs to existing blocks on the main arch are legible. The original stone-cut channel for the creek still exists but is obscured by a concrete channel poured within Lennox's work during the 1976 repairs. Since Mitchell's Pass ceased to take the heavy traffic over the Mountains, via the former Knapsack Railway Viaduct, the road is now one-way only downhill from Lennox Bridge and the traffic is largely for tourist purposes. There is a small amount of car-parking space beside the bridge and a concrete stairwell and path down to the creek on the west side. The visitor can safely walk under the bridge obtain the most attractive views.
The road was closed, the carriageway removed and the bridge stripped down to its bare stone arch in 1976. The Department of Main Roads decided that the stress had to be removed from the original arch and reconstructed the carriageway so that it was supported on two new concrete walls and borne on horizontal steel girders. With other repairs to the stonework and to the channel for the creek under the bridge, the road was reopened in 1976 with the new work largely concealed from view. With much reduced traffic flow and the road to the plains from the bridge made on-way only (downhill) Lennox Bridge continues to serve as the oldest stone bridge on the Australian Mainland. (Kullas, LGEA of NSW Journal, July 1983, 85- 86).
Physical condition is good.
Mr David Lennox, who left his stone wall at my request, and with his sleeves still tucked up, came with me to my office, and undertook to plan the stone bridges required, make the centring arches and to carry on such works by directing and instructing the common labourers then at the disposal of the Government.’ (Major Mitchell, Letter to Perry, March 22, 1833)
And on 1 October 1832, only seven weeks after his arrival in the colony, Lennox was appointed (subject to London's confirmation) Sub-Inspector of Bridges in New South Wales. His first task was to plan and organise the spanning of Lapstone Creek on Mitchell's Pass. By early November 1832, he and his selected twenty convicts; with suitable experience had opened a quarry near the creek, had cut a number of stone blocks and were ready to start building operations. The design called for a horseshoe shape to give optimum strength.
By March 1833, the experimental bridge was so far advanced that Mitchell had to decide what should be carved on the keystones. He agreed that on the upstream side (the south side), the inscription should commemorate the man he had chosen so percipiently, so the masons carved 'DAVID LENNOX', while on the north side they carved 'AD 1833'. There has been weathering on the south side but the AD1833 stone is still in very good condition. (Selkirk, Journal of Royal Australian Historical Society, V6, 1920, P 202-207).
A small crack appeared beside the arch but Lennox advised against taking any remedial action. The bridge remained very robust and carried increasingly heavy traffic as the Great Road to the West became busier. The success of the railway after 1867 diverted much transport business away from the road until the twentieth century and advent of the motor-car. Mitchell's Pass began to bear more traffic than was comfortable for Lenox Bridge, although the conversion of Knapsack Viaduct from a trail line to a road bridge in 1926 opened a new road route up the Monocline and at last diverted much of the traffic from Mitchells Pass.
Mitchell's road continued to be used, however, and by 1975 it was plain that substantial works were necessary to secure the safety of Lennox Bridge. Restoration works took place in 1976.