OCR Text |
Show Rocks Made To Tell Their Own Story Tho walla of tho Grnnd Canyon in Arizona form a great natural geologic section, in which each layer of rock In In Its original position relative to those' above and iiolow it. in a row other nluces, howover Is tho story of tho unbuilding of tho earth's crust so plainly and lmpiesslvely told. As u rule t;ho geologist who would doclph cr tho records of tho rocks must get a bit? hero nnd a bit there Ho mny find tho edges of some beds oxposcd In a irlver bluff and others sticking out on a steep mountain side. Ho determines by fossils or other means tho order Irt which tho beds were do-posltqd, do-posltqd, and by putting all his Information Inform-ation together ho constructs what ho calls 'a, columnar section for tho district dis-trict .In which ho is working that Is, a section showing tho order, tho thickness, nnd character of tho beds. Such a section discloses tho .strata that, form tho upjjor part of tho earth's crust at that'placo, Just as a slice of layer cako, shows at a glance the vb.-rloiiB vb.-rloiiB layero of which it Is composed. Aftor n number of districts In a legion have been studied and their general columnar bcctions dctcrmln ed, tho geologic history of tho legion can bo learned by comparing theso nectloiiR, Just an tho engineer whe is drilling ior low grndo topper ores compnicn lits drill records nnd thus lcanib tie outlines of the nro bod) Such n cunipmlsoii ol tho beds nt ono plato iiii'l mother shows how certain beds thango In diameter and thickness thick-ness fiom ulace to place or even th'n out and disappear It eimhkrf tho geologist to ilnnv rwimn conclusions .ih t, tho former distribution of land nnd fen, to distinguish tho deports laid down In deep water fiom those spread by rivers over tholr Hood plains nnd to icconstiuct in imagination imagina-tion the courto of events nt u tlmo long boforo tho beginning of tho Grand Canyon. Such a comparison Iioh rccentlv been made for Arltoim and Is published by tho United Stales Geological Survey, Department of tho Interior, bb Professional Paper US K, by F. L. Rnnsomo, geologist. Tho report Is obtainable on request from tho Director. |