Page:Early Man in Britain and His Place in the Tertiary Period.djvu/292

264 river-valley, where both are close together; the Pleistocene fluviatile strata occurring in various levels, either above or below the present level of the stream, while the Prehistoric deposits consist of alluvia close to the present level of the stream, or of subaërial accumulations of loam and the like, the result of the rain-wash, covering the lower grounds like a mantle. In the former the severity of the winters is marked by the confused manner in which the pebbles have been accumulated, owing to the floating ice in the streams; while in the latter the sediments are sorted by the ordinary action of running water, without the intervention of ice. The line of demarcation is equally clear in the caverns, in which the late Pleistocene accumulations are generally mapped off from those of the Prehistoric age by a layer of stalagmite, sometimes of considerable thickness. This, however, offers no measure of the interval between the two periods, because the rate of accumulation depends upon the currents of air in the caves, and the amount of water passing through the limestone, both of which are variables. In the Ingleborough cave, in Yorkshire, it has been so swift that between 1845 and 1873 a stalagmitic boss known as the Jockey Cap has grown at the rate of ⋅2941 inch per annum. In Kent's Hole it has been so slow that an inscription bearing the date of 1688 on a similar boss is only covered by a film not more than one-twentieth of an inch in thickness. It therefore follows that very great thicknesses may be formed in a short time; while on the other hand it may take a long series of centuries to form a thin layer of a few inches.