Depedale before the Priory
Rather more than 300 million years ago, Britain was just crossing the Equator. Under a shallow tropical sea, coral reefs had formed; but the sea sank, leaving a swampy lagoon with forests of giant fern. These swampy sediments collapsed under the silt and were crushed by later rocks above, forming the Upper Carboniferous Coal Measures found in the area now known as south Derbyshire.
Later, the sea rose, or the land sank, or both, and a huge river delta covered this region; its coarse sands formed the Upper Carboniferous Millstone Grit Series, which outcrop and form the higher undulating ground north-east of Derby.
Those ‘later rocks’ mentioned above include the red sandstones and mudstones of the Triassic age, around 248 to 205 million years ago. These were formed in dry, arid, desert-like conditions, responsible for the oxidation of iron compounds within the rocks. This accounts for the characteristic red colouration of these deposits. The Triassic succession of the overlying Mercia Mudstone Group, which comprises red and green mudstones, sandstones and thin horizons of gypsum and rock salt, probably represents wind-blown dust that settled in shallow salt-lakes and sun-baked mudflats.
In more recent times, namely the Quaternary (1.8 million years ago until the present time), these coal mudstones and sandstones were covered by glacial deposits. Large amounts of sand, gravel and boulder clay occur in the nearby Erewash Valley; these were deposited by ice sheets and meltwaters over the past 500,000 years.
The result of all this, geologically speaking, is that most of the southern part of Derbyshire is occupied by Keuper marls and sandstones, while red beds of Bunter marl, sandstone and conglomerate appear at nearby Morley and around the south Derbyshire coalfield. Bunter sandstone has been quarried in the past at Stanton by Dale.