Excerpt from Memoirs of the Geological Survey, England and Wales: Explanation of Sheet 338; The Geology of Dartmoor

The following pages describe the geology of an area of 216 square miles in Devon, including the greater part of Dartmoor. NO coast line or tidal estuary cuts into this district, and the Moor, which is the highest land in south-western England, dominates everything. The geology we have to deal with is essentially the geology of Dartmoor; for the rocks outside the actual borders of the granite are either baked by the granite, or have been thrust aside by the intrusion of this granite.

Large masses of Dartmoor rise feet above the sea and much of it is over at several points it exceeds feet, but the only tors reaching happen to lie just beyond our border. The highest points within the map described in this Memoir are Cut Hill, which reaches feet, and Whitehorse Hill, which rises to feet. These two hills, two miles apart, represent the watershed and central area, from which the rivers radiate north, south, east, and west; the. Somewhat higher tors further north are not on the watershed.

This central and northern area is also the wettest and most peaty part of Dartmoor, for it has an annual rainfall of about 60 inches. All the land above feet rises into the region of clouds; so that not only has it an exceptionally heavy rainfall, but the constant mists and slight evaporation tend to make it a far more efficient feeder of the streams than its area alone would suggest.

The wide-spread upland peat-mosses of Dartmoor in which the rivers rise occupy most of the land above feet, t.e., the area north-west of the Princetown and Moretonhampstead road. These mosses form continuous sheets clothing the highest land, and in this they are quite unlike the basin-peats of the other and lower granitic areas in Devon and Cornwall. The growth of the peat is directly connected with the elevation; for the land above feet, as already mentioned, is liable at all times of the year to be wrapped in cloud. Thus a peaty vegetation grows, and peat accumulates on Slopes that 'at any lower elevation would show merely a rough boulder-strewn granitic soil.

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