Mam Tor is a classic landslip mountain, known locally as 'The Shivering Mountain'. The rock is so unstable that even the main road from Sheffield over to Lancashire eventually became unusable. BGS: Mam Tor Beds - Siltstone And Sandstone. Sedimentary Bedrock formed approximately 315 to 316 million years ago in the Carboniferous Period. Local environment previously dominated by sub-aqueous slopes