Annual report of the Bureau of Ethnology to the Secretary of the Smithsonian Institution . he sedimentaiies and resting on the beds of bowlders are found higher in this section, but nonehappen to be so well suited to the use of the primitive implement makeras those representing the work of the waves along the crystallinebeach. The surface of the gneisses was somewhat uneven, slopinggently beneath the waves, and the bowlder beds laid down on this sur-face are of uneven thickness and not of uniform character when fol-lowed out horizontally, coarseness decreasing with distance from
Annual report of the Bureau of Ethnology to the Secretary of the Smithsonian Institution . he sedimentaiies and resting on the beds of bowlders are found higher in this section, but nonehappen to be so well suited to the use of the primitive implement makeras those representing the work of the waves along the crystallinebeach. The surface of the gneisses was somewhat uneven, slopinggently beneath the waves, and the bowlder beds laid down on this sur-face are of uneven thickness and not of uniform character when fol-lowed out horizontally, coarseness decreasing with distance from theriver channel. The aboriginal inhabitant, seeking for stone suitablefor his use, discovered these outcrops of bowlders along the bluffs ofthe Potomac and its tributaries, and soon ascertained that the depositswere heavier and the quality of the material better and more uniformin Rock creek valley than in any other section. This discovery led intime to subterranean search on the more favorable sites and finally toextensive quarrying, tlie evidences of which are now brought to Fig. 2—Section of the ravine, showinj; formations and position of quarries. Owing to the friable nature of the bowlder beds and of the gravelsand sands overlying them, the terrace slopes bordering the streams(save where erosion had recently been particularly active) offered nogood exposures of the bowlders in place, but were covered with depos-its, often many feet in thickness, of gravelly talus derived from thecrumbling edges of the strata. The bowlders contained in this over-placed deposit were the first to be utilized, and the work then extendedto the bowlder beds proper, and the of the quarrying was addedto the creeping slope gravels or talus. The section given in figure 2 shows the relation of the gneisses, thebowlder beds, and the superficial deposits of sand and gravel outcrop-ping in the quarry ravine. PINY 13KANCH yUARRIES Location of the Quarries In passing out of the city by
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