The story of how the canal eventually got built begins, oddly, with a railway. In 1821, directors of the new Plymouth and Dartmoor horse tramway needed permission to deviate their route. To get John Parker's son, by then the Earl of Morley, to agree, they promised a branch line to his quarry. The company hit financial trouble and the branch was never built. Morley sued, lost the argument, and the directors made him a counter-offer: build the branch yourself, and we'll give you preferential tolls on the connection. Morley dug in his heels. He decided to build a canal instead. A six-foot-wide tub-boat canal would carry the slate down the valley to Marsh Mills, where it would meet either the turnpike road or the tramway, his choice.

The canal opened on 20 November 1829. It ran for just under two miles. A short half-mile tramway carried cargo the final stretch to Crabtree Junction, crossing the River Plym on a graceful two-arched cast iron bridge that still stands. The whole arrangement worked, after a fashion. But within a decade, Earl Morley had extended his own tramway up the valley, running it alongside and sometimes on top of the canal bank itself. By 1839 the canal had ceased to function as a navigation. The water that had carried slate barges was rerouted to drive the wheel at Marsh Mills corn mill instead, a job it would do quietly for the next ninety years until the mill's owners went bankrupt in 1927.

Crabtree Junction is gone now, buried under the interchange where the A38 meets the A374 and a motel parking lot. But Morley's two-arched iron bridge survives. The old tramway has become the West Devon Way, a long-distance footpath and cycleway that follows the river up into the woods. Most of the canal itself is still visible, a narrow channel running parallel to the path, often filled with rainwater, framed by ferns and moss. The Plym Valley Railway, a heritage line on what was once the South Devon and Tavistock branch, is being extended toward Plym Bridge. Walk the route on a wet day and you can see all of it at once: the quarry that started the argument, the canal that lost it, and the tramway that won.