The Alport Castles are a landslip feature in the Peak District National Park in Derbyshire. At over half a mile long, it is thought to be the largest landslide in the United Kingdom.[1] The name "castles" comes from the debris from the landslide, which has produced several gritstone mounds that tower over the valley and appear from the distance to look like castles. Viewed from a distance the largest of these, the "Tower", resembles a full-scale motte and baileycastle.
300 million years ago, the area now known as the Peak District was part of a river delta that flowed into the sea.[1] The deposits were sorted such that the finest material travelled the furthest and was deposited in the deep ocean as black shales. Further deposits accumulated on the slopes of the oceans and collapsed, resulting in turbidite deposits. Further turbidite flows eroded into previous ones, resulting in the type of deposit seen at Alport Castle. As the delta prograded (the mouth of the river moved further out to sea), the deposits become coarser. In the Peak District this coarse material is the gritstone that caps high points, protecting them from erosion.
The exact cause of the landslide is still debated. One theory is that the soft shales below are too weak to support the weight of the heavy sandstone above and collapse under it, or that, because water can run through gritstone but not shale rock, trapped water may have "lubricated" the rock to the point where one layer slid over another, causing the landslide.[2] A further possibility is that a valley glacier steepened the sides of the valley, leaving unstable slopes that failed after the glacier melted, causing the landslide.[2] However, immediately upstream is a normal river valley so any glacier would have been small.
Alport Castles has been selected for geological conservation as one of the most significant landslips in Britain.[3]
Today
The rock faces and cliffs are unstable and unsuitable for climbing and scrambling; however, the site is accessible along some well-trodden public rights of way, and is a popular site for walkers.[1]
The site is also of interest for birdwatchers, as both ravens and peregrine falcons have been known to nest on the crags.[1]
The remote Alport Castles Farm lies on the River Alport below the site. This is the farm where the suffragette Hannah Mitchell was born in 1871 and brought up.[4]
