GEOLOGY Erewash valley. South of the Trent it is found at Repton and Fore- mark. In the neighbourhood of Ashbourne the Bunter is much obscured by drift, and its boundary is somewhat uncertain. The uncertainty of arriving at the true boundary of rocks in a country covered by a mantle of drift is exemplified in the district immediately south of Ashbourne. A well was sunk for the Ashbourne waterworks on Spital Hill where the ground was mapped as Bunter by the Geological Survey. But 70 feet of drift and over 60 feet of Keuper were passed through before the Bunter beds were reached. Though the Bunter is seen at the surface at many places it is not always present under the Keuper. At Snelston Common the Keuper Marl rests on the Mountain Limestone, and at Mickleover a borehole passed from the Keuper into the limestones and shales of the Yoredale series ; whilst at Derby, 4 miles distant, another borehole indicated that the Bunter was probably present under the Keuper. The Bunter Sandstone consists of a soft yellow and light red sandstone with scattered pebbles mostly of quartzite. The proportion of pebbles to sand varies considerably. Where the pebbles are numerous the rock becomes a gravel with little sand and when hard a conglomerate. The Bunter in Derbyshire is considered to belong mainly to the pebble beds or conglomerate, the Lower Mottled Sandstones being absent except at Dale. Good exposures of the Bunter beds are seen at Ashbourne, where they are at least 120 feet thick. The road from Ashbourne to Clifton is cut through beds of a soft yellow and light red sandstone containing a few pebbles. These measures are also seen in the banks of the old Derby road south of Ashbourne, at Bradley Wood and near Sandybrook Hall. At Brailsford, 23 feet of the pebble beds are exposed in a quarry, and the conglomerate is well exposed on the road from Repton to Ingleby. Near Sandiacre the rock is quarried. It consists mainly of a soft yellow and light red rock with few pebbles. Some of the beds are sufficiently hard to require blasting, but soon become friable. The sand is used largely for building. The Keuper beds (or New Red Marl), which overlie the Bunter, are in this district divided into the Red Marl and the Waterstones or White Beds. They occupy a large tract of county south of Ashbourne, Mug- gington, Breadsall and Sandiacre, and stretch across the southern part of the county in a direction from east to west. On the west and south they are bounded by the Dove and the Trent, and on the east by the Erewash. The Upper Keuper consists of beds of red marl and shale, with intercalations of greyish, sandy and micaceous shales or sandstones (called skerry) and irregular bands of gypsum, a hydrated sulphate of lime. The marl is largely used for making bricks and forms a stiff soil well suited for agricultural purposes. The gypsum, which is burnt for making plaster of Paris, occurs in irregular beds, spheroidal masses, lenticular intercalations and veins in the marl. The beds thin out and come in again in a short distance, and vary in thickness and quality. At Chellaston and Aston the gypsum occurs in a bed about 10 to 15 feet in thickness. At the latter place it is worked underground. A tough 27
