New moon: in the early hours of this morning a woman drove the wrong way up the A38. After two miles she hit a car coming the right way down the dual carriageway, and killed herself and the two people in the other car. There's diversion all day at Deep Lane. The road's just been cleared of wreckage and reopened.

After college we drive to Marsh Mills roundabout at the head of the Laira. We walk out of Sainsbury's carpark under the flyover. It's high tide and just falling. Fresh and salt water mixed with china clay run-off has swollen the river till hemlock flowers along the banks are drowned and there's a muddy scum line in the grass.

We follow the number 27 cycle path beneath the dual carriageway, over the bridge by the Speedway track, then back under the flyover and along the river Plym as it curls round to Saltram woods and becomes an estuary.

Marsh Mills is a crossover point where many routes intersect. The railway line cuts under the A38 and over the river: the trains carry passengers and freight along a single track laid by Brunel. Roads meet from every direction here; people walk dogs, jog and cycle. The enormous concrete cave under the flyover is a hangout for youth. The smoothed earth mound in the centre is an omphalos built by BMX tricksters; the pillars, ramps and girders are patched with graffiti: sprayed paint layers and market pen. I recognise tags I saw during the summer of 2000 when we filmed Shop Around the Clock near here.

To and fro. The noise of people, traffic, trains, voices, echo round the great chamber. Birds flying near dusk, goldfinches and ducks. A rat on the tarmac path. Diesel smoke, scarlet pimpernel, purple orchids, scrub. On the east side of the Laira we wade through nettle and brambles in the beech wood, then hogweed and hemp nettle, along the bank of the flooded meadow till we're at sea level, water all around. Dead birch leaves swirl downstream. In the air there's a tang of chlorophyll and salt, the taste of sour green almond.