My life : a record of events and opinions - Wallace-online.org

90 MY LIFEeverywhere intersect it, must be all due to sub-aerialdenudation— that is, to rain, frost, and snow—thedibris due to which is carried away by the brooksand rivers. The geologist looks upon the roundedsummits of these mountains as indications of anextensive gently undulating plateau, which had beenslowly raised above the surface of the lakes or inlandseas in which they had been deposited, and subjectedto so little disturbance that the strata remain in anearly horizontal position. When from the summitof any of these higher mountains we look over thewide parallel or radiating valleys with the roundedgrassy ridges, and consider that the whole of thematerial that once filled all these valleys to the levelof the mountain-top has been washed away day byday and year by year, by the very same agenciesthat after heavy rain now render turbid every brooklet,stream, and river, usually so clear and limpid, weobtain an excellent illustration of how Nature worksin moulding the earth's surface by a process so slowas to be to us almost imperceptible.This process of denudation is rendered especiallyclear to us by the singular formation of the twinsummits of the Brecon Beacons. Here we are able,as it were, to catch Nature at work. Owing to therare occurrence of a nearly equal rate of denudationin four or five directions around this highest partof the original plateau, we have remaining for ourinspection two little triangular patches of the originalpeat-covered surface joined together by the narrowsaddle, as shown in the sketches opposite, giving aplan of the summits and a section through them toexplain how accurately the two coincide in theirslope with that of the original plateau. Every yearthe frost loosens the rock on the northern precipices,