In an amazing getaway the Guernseyman, riding for Trinity Racing, made an attack with 107km to go and stayed out front solo, increasing the gap to three minutes at one point.
He led from the peloton for two hours and 96 kilometres before being caught by the main group with just 11km to go.
Adding further testament to his performance, it would appear seems that the whole team of the yellow jersey rider had to sit on the front to drag him back in the end.
Being in the front he took all three King of the Mountain climbs.
His solo attack meant that the rest of his team did not have to do any work in the peloton.
Previously, on the opening 120km stage from Illiers-Combray to Brou, the Guernsey youngster placed 29th in the 140-plus field after close to three hours’ racing, given the same time as the frontrunners in the mass sprint.
In Saturday’s 186.2k second stage from Courville-sur-Eure to Dreux he was again well placed, finishing 31st and just eight seconds behind the large group that contested the sprint finish.
Yesterday’s stage was the longest, a flat 193.5k roll out between Bonneval and Chartres.
Culverwell and the six-strong Trinity Racing team were competing against 22 other teams, including many crack development squads.