A fancy-sounding London street is said to have inspired our local worthies more than a century ago to drive the town into the country. They paved paradise past the mill, laying down a hard-standing path alongside the river, edging it with hooped iron railings, the latest in urban parkland chic.

The promenade they called Birdcage Walk then cut inland, stealing the border between copse and water meadow, before swinging back to cross the river again on a footbridge at the railway end of town.

Birdcage Walk in Sandy, Bedfordshire. ‘They paved paradise past the mill, laying down a hard-standing path alongside the river…’ Photograph: Sarah Niemann

Nature here has never really reconciled to being slathered in a blanket of bitumen. A wayside ash has been a longstanding rebel, raising the stamped-down ground with its burgeoning roots, creating pleated waves that have broken the back of every path laid. The authorities tried resurfacing last winter for the umpteenth time. It has failed to quell the protest. Soft plant tissue with diamond-cutting tips has cracked, split and sprouted through the tar this summer as little green volcanoes of thistly lava. The path is littered with seasonal showers of blackthorn blossom, willow down and alder cones, but they cannot mar its ugliness.

On a morning after overnight rain, Birdcage Walk had become a perilous crossing, a fast-drying river for thousands of snails that slid from field to copse, and copse to field. There were pretty penny-sized coils of cream, lime green or pink; zebra-striped banded snails; and nut-brown garden snails, the giants among these migrants of the damp.

‘Soft plant tissue with diamond-cutting tips has cracked, split and sprouted through the tar this summer as little green volcanoes of thistly lava.’ Photograph: Sarah Niemann

Soon there would be crocodiles of a sort on this hard river, the heavy feet of commuters pounding their way towards the station, crushing underfoot those that were caught in the open, the smaller snails making guilty popping sounds, the bigger specimens trod with gravelly crunches. For now, though, there was a purposeful killer at large. The previous day I had seen a startled song thrush dart into the bushes at a point on the path where there was a scatter of broken shell. Here it was, at the exact same spot, only this time with a big snail hanging from its beak. Though it disappeared in a leggy sprint for the undergrowth, it would surely be back. This was perfect hunting and an anvil several hundred metres long.