Opened in 1970, Scammonden is unique in Britain, the only dam that both holds back the waters of a reservoir (Scammonden Water) and carries on its crest a motorway (the M62). It's reached by exiting at junction 23 and taking the A640 over Saddleworth Moor.
Excavation commenced in 1964, the first task being the removal of 40 feet of soft peat, which characterises the area. The dam has a clay core, and a graded sandstone fill. The overflow bellmouth is rather like that at Ladybower Reservoir, outside Sheffield (YMGW passim). A number of access points enable engineers to enter the dam's bowels.
Motorways take up a lot of space. The dam is 60 yards wide at the top, and 475 yards wide at its base. Its scale can be appreciated by passing through the subway, large enough for a mid-size truck, that runs through the embankment. A club hillclimb track, operated by Mid-Cheshire Motor Racing Club, snakes up the east flank.
Another engineering marvel is nearby - Scammonden Bridge, carrying a B-road over the Deanhead cutting, through which the motorway passes before/after traversing the dam. When built, this was the longest concrete single span bridge in the world, with an arch of 140 yards and a deck of 220 yards.
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