The story of the land of Somerset as we know it now must begin somewhere in the first quarter of the second millennium of the pre-Christian era. By about 1700 b.c. the surface of south-west England had assumed much the same features as it has today allowing for subsequent slow alterations of altitude by elevation or depression and the constant denudation of hills and silting up of valleys. The exposed strata were generally the same as the modern geological map reveals. The limestone beds of the Carboniferous system form the Mendips and other detached hills towards the Severn on a curve some fifteen miles west and south of Bath. To the east and north they are overlaid by the coal-measures of the Bristol and Somerset coal-fields. In the upper valleys of the Axe and the Brue and in the lower valley of the Parrett the Rhaetic beds of the Triassic system come to the surface, but the lower levels of these valleys are now for the most part covered by alluvium. The alluvium of the Yeo and the Ile rests on the Lower Lias beds of dark blue clay and limestone, the eastern and southern boundaries of the exposed surface in the county passing near Shepton Mallet, Castle Cary, Sparkford, Mudford, Tintinhull, Martock, Kingsbury Episcopi to Ilminster and on to the west.