Geology . k. m Fig. 491 .—A polished surface of rock in Bronx Park, X. Y. (Willis, U. S. Geol. Suit.). Fig. 492.—Roches moutonnees. THE PLEISTOCENE OR GLACIAL PERIOD. 353 rock formations and of topography. From its physical make up weknow that the agency or agencies which produced it must have beenable to carry and deposit, at one place and at one time, materials asfine as the finest silt or mud, and bowlders many tons in weight, whilethey were competent, under other circumstances, to make depositsof much less extreme diversity. From its lithological make up, andfrom the nature of the finer pa
Geology . k. m Fig. 491 .—A polished surface of rock in Bronx Park, X. Y. (Willis, U. S. Geol. Suit.). Fig. 492.—Roches moutonnees. THE PLEISTOCENE OR GLACIAL PERIOD. 353 rock formations and of topography. From its physical make up weknow that the agency or agencies which produced it must have beenable to carry and deposit, at one place and at one time, materials asfine as the finest silt or mud, and bowlders many tons in weight, whilethey were competent, under other circumstances, to make depositsof much less extreme diversity. From its lithological make up, andfrom the nature of the finer parts of the drift, we know that the driftforces worked on different sorts of rock, deriving materials from many;that they groimd some of the materials into a fine earthy powder orrock flour/ commonly called clay; that they as a rule derived thelarger part of the drift of any locality from formations near at hand;and that the materials, even large bowlders, were sometimes carriedup to altitudes considerably above their source. From the structureof the drift it is concluded that the drift force or
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