In Derbyshire the Millstone-grit series consists of four or five well-marked grits, separated by shales, viz. the so-called first or topmost grit, named, from its coarse character, the rough rock; the second grit, which is generally a flagstone; the third grit, a bold, well-jointed rock; and lastly, the Kinder Scout grit, which sometimes consists of two beds. In advancing northwards this type undergoes considerable changes; the second grit becomes merely a basement bed to the rough rock, not always separable therefrom; the third grit loses its massive character, and other beds of sandstone begin to show themselves amid the shales overlying the Kinder Scout grit. When one reaches the valley of the Colne, four separate sandstones have developed themselves between the rough rock (locally known as the sand rock) and the recognized Kinder Scout grit. This type prevails also in the valley of the Yorkshire Calder; but is not to be found on crossing the watershed into the basin of the Aire. There the series consists in descending order, first of the rough rock, which throughout maintains its usual marked character till it is buried beneath the Permian; secondly, of a very variable basement bed to the last, consisting, when well developed, of valuable flagstones. These are extensively quarried at Nab, above Oxenhope Moor; and also in an outlier at the Penistone quarries, near Haworth. Below this bed comes a fresh series of variable sandstones and shales. There may be in places as many as fifteen or sixteen distinct sandstones below the basement of the rough rock and the regular Kinder Scout grit.