Editing Mam Tor (section)

==Landslides==
[[File:SubsidedRoad.jpg|thumb|Section of the abandoned road]]

A feature of Mam Tor is the active [[landslide]] which invades its southeast side almost to the summit, and interrupts the ramparts of the hillfort, unless its builders used it as part of the defences. This rotational landslide began roughly 4,000 years ago. The toe is a [[debris flow]]. The landslide is due to weak [[shales]] underlying [[sandstones]], a common phenomenon all around the Dark Peak, notably at [[Alport Castles]], [[Longdendale]], [[Glossop]] and [[Canyards Hills]], Sheffield. Indeed, three larger landslides occur on the north side of Mam Tor, one of them cutting the main ridge at Mam Nick which allows a minor road over into Edale; another creates the striking crag of Back Tor well seen from Mam Tor.<ref>{{cite book |last1=Cooper |first1=R. |last2=Jarman |first2=D. |date=2007 |chapter=Mam Tor, Derbyshire |title=Mass Movements in Great Britain (Geological Conservation Review Series No. 33) |publisher=JNCC |pages=167–183 |url=https://data.jncc.gov.uk/data/c9bdfe0b-ca6f-4a90-87a6-d03f64e8124c/gcr-v33-mass-movements-c5.pdf |isbn=978-1-86107-481-2 |location=Peterborough |access-date=20 November 2024}}</ref>

Evidence for the continued movement of the slide mass is demonstrated graphically by the severe damage to the old [[Mam Tor road]] that traversed this flow. The road was built at the beginning of the 1800s and was subsequently relaid until local authorities closed it in 1979. Layers of tarmac and gravel are up to {{convert|2|m}} thick in places, demonstrating the numerous efforts to keep the road open. A short tunnel could readily have been made avoiding the landslip zone, but the opportunity to exclude heavy business and lorry traffic from the middle of the National Park was preferred. A local diversion for light vehicles follows the limestone gorge of Winnats Pass. This is one of the most extreme cases of geological problems affecting main transport systems in Britain, comparable with the railway at [[Dawlish#Transport|Dawlish]].

Current mean annual movement is "up to 0.25 m; this increases greatly when winter rainfalls exceed thresholds of both 210 mm/month and 750 mm in the preceding six months".<ref>{{cite journal |last1=Waltham |first1=A. C. |last2=Dixon |first2=N. |date=2000 |title=Movement of the Mam Tor landslide, Derbyshire, UK |journal=  Quarterly Journal of Engineering Geology and Hydrogeology|volume=33 |issue=2 |pages=105–123 |doi=10.1144/qjegh.33.2.105|bibcode=2000QJEGH..33..105W }}</ref> The debris flow poses no threat to any inhabited buildings; however, small farm buildings lying in the flow's path may soon be overwhelmed assuming a flow rate similar to that of the present. The 2000 study suggests that deep drainage may be the most effective means of stabilising the flow, though this may not completely stop movement.