One of the outcrops known as "The Tower"
Alport Castles shown within Derbyshire
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. The name "castles" comes from the debris from the landslide, which has produced several protruding grit-stone mounds which 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 bailey castle.
The Alport Castles are situated on the Eastern side of the River Alport Valley, and is part of the National Trust's High Peak Estate; they lie north of the Snake Pass and north west of Ladybower Reservoir.
The exact cause of the landslide is unknown, but similar if less dramatic landslips occur all around the Dark Peak, notably on Mam Tor. 300 million years ago, the area was part of a river delta that flowed into a shallow tropical sea which covered the area now known as the Peak District. Over millions of years the mud and sand deposited by the river compressed to form the layers of soft shale and hard gritstone rocks which can be seen today.
One theory is that the river eroded the softer layers, causing the landslide.
Another theory is that because water can run through gritstone but not shale rock, trapped water may have "lubricated" to the rock to the point where one layer slid over another, causing the landslide.
A further possibility is that a valley glacier steepened the sides of the valley leaving unstable slopes which failed after the glacier melted, causing the landslide. 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.
The remote Alport Castles Farm lies on the River Alport below the site.
Media related to Alport Castles at Wikimedia Commons