The reserve is also part of the Royal Society for the Protection of Birds Northward Hill SSSI site which includes 270 hectares of grazing marsh, woodland and farmland. The area features the largest heronry in Britain, where some 150–200 pairs nest. A heronry has been in the woods from at least 1947, when it was mentioned in a book on the Hundred of Hoo by Ralph Arnold. High Halstow NNR was declared a National Nature Reserve in 1951. The Thames and Medway Canal The Rivers Thames and Medway were joined by the eight-mile long Thames & Medway Canal, the construction of which started in 1805. With basins providing ports and access to the two rivers at Gravesend and Strood, the canal passed through a two-mile tunnel at Higham, broken in the centre by a shaft to allow boats to pass. Construction was difficult and expensive and it was not completed until 1824. Shortly afterwards, the tunnel became part of the railway linking Higham with Strood. The line of this now-silted canal can be considered a convenient boundary marking the landward edge of the peninsula. The tunnel passes through large beds of chalk. To cut engineering costs many sections were not lined, which caused various rock falls between 1957 and 2004. As these rock falls could cause derailment, the tunnel was closed to trains during 2004 and was relined with reinforced concrete. It reopened in early January 2005. Roads The only main road is the A228, which crosses the old Roman London Road (also called Watling Street, now the A2) at Strood and then follows the high ground eastwards. It meets the Medway Towns Northern Bypass (A289) at the bottom of Four Elms Hill and climbs to Chattenden, bypassing Hoo St Werburgh and High Halstow, before crossing to the Isle of Grain. On the Peninsula this road is known as the 'Ratcliffe Highway'. The other principal route on the peninsula, the B2000, heads north to Cliffe on the edge of Cliffe marshes, part of the North Kent Marshes. This is a winding country road, much used by industrial transport serving the larger farms, including Mockbeggar Farm, and the industrial jetties onto the River Thames. The B2000 passes through Cliffe Woods under the name of 'Town Road', and enters Cliffe, where it becomes first 'Station Road', from the location of the now vanished station of the Hundred of Hoo railway, and then