On Limitations to the Use of Some Anthropologic Data by John Wesley Powell

The pictographs of North America were made on divers substances. The bark of trees, tablets of wood, the skins of animals, and the surfaces of rocks were all used for this purpose; but the great body of picture-writing as preserved to us is found on rock surfaces, as these are the most enduring.

From Dighton Rock to the cliffs that overhang the Pacific, these records are found -- on bowlders fashioned by the waves of the sea, scattered by river floods, or polished by glacial ice; on stones buried in graves and mounds; on faces of rock that appear in ledges by the streams; on cañon walls and towering cliffs; on mountain crags and the ceilings of caves -- wherever smooth surfaces of rock are to be found in North America, there we may expect to find pictographs. So widely distributed and so vast in number, it is well to know what purposes they may serve in anthropologic science.