Page:VCH Derbyshire 1.djvu/79

 GEOLOGY latter resting on the Keuper Marl with gypsum. The patch of Chalky Boulder Clay at Chellaston fills up an old valley and caps the hilltop. It is of a sedimentary character, and has to a great extent escaped the grinding character of the ice. The boulders range in age from Carboniferous to Cretaceous, and the Carboniferous rocks have been derived from the Older Pleistocene Boulder Clays. East of Chellaston the great Chalky Boulder Clay is not met with again till we come to the ridge between south Notts and Leicestershire. In Derby a patch of stratified gravel with flints was seen in an excavation in Green Lane. It probably belongs to the Chalky Gravel stage, as does also another patch near the Arboretum. On Chellaston Hill the Chalky Boulder Clay is capped by a deposit of gravel at least 17 feet in thickness and covering an area of about a quarter of a square mile. A section of this sand and gravel may be seen between Chellaston and Weston. The sand contains flint and quartz pebbles. On the high land south of Ashbourne gravel is of tolerably frequent occurrence, in most cases the deposit is very much disturbed and signs of stratification are only shown in the lower parts of the sections. The deposits of Newer Pleistocene age indicate the first signs of subaerial erosion and the consequent formation of river gravel. During this stage the rivers cut down their valleys through the older Boulder Clays and sands to within about 20 feet of their present depths and left their gravels stranded as terraces at various heights above their present courses. Upon these gravels there frequently rests a Boulder Clay which is conformable to the surface features produced by the erosion of the previous stage. The erratic boulders distributed over the westerly part of Stafford- shire are considered by Mr. Deeley to belong to this stage and to prove that the climate was sufficiently severe for the Scotch and Cambrian glaciers to invade the western portion of the Trent basin. The interglacial river gravel occupies terraces at various heights along the valley of the Trent and its tributaries. In the valley of the Trent between Findern and Weston there are long patches of high level river gravel, which by their oblique bedding indicate currents down the valley. A lower terrace may be traced from near Weston to within about half a mile of Aston, and a still lower series of terraces occupies considerable areas in the valleys of the Trent and Derwent. One of these, a large crescent shaped terrace, stretches from Willington to Stenton Lock. Its escarpment runs along the north side of the Trent valley past Swarkestone, Weston, Aston and then up the valley of the Derwent past Elvaston and Osmaston to Derby. Posterior to these river gravels is the later Pennine Boulder Clay which is well developed in south Derbyshire. Wherever the rocks upon which this clay rests are exposed they are seen to be contorted. Mr. Deeley considers that the contortions have been formed by the same ice sheet that produced the later Pennine Boulder Clay, and that the direction in which the ice sheet moved is