9 INTERIOR/GEOLOGICAL SURVEY USGS· INF -72- 2 !R I) - Electric City-Grand Coulee, Washington THE CHANNELED SCABLANDS OF EASTERN WASHINGTON - The Geologic Story of the Spokane Flood- '(( U.S. GOVERNMENT PRINTING OFFICE , 1976 0 -208-172 F'or sale by the uperint.endent. of Documents, .S. Government Printing Office Washington, D.. 20402- Pric~ 70 cents Lock No. 024 - 001-02507- I nL nlog o. I 19.2: W27/6/974 There is n minimum charge of $1.00 for each mail order A trave ler enterin g th e tal f W as hington fro m th e Eas t crosses a flat-to-rolling country­ sid o f deep, fertil so il commonly sown with w heat. ontinuing wes twa rd , he abruptly nters a d eply scar red land o f bare bl a k ro k ut by labyrin thine ca nyons and chann el , plunge pools and ro k bas in s, ca ca de and ca tara t ledges, and di playi ng ragged buttes and li ffs, alcoves, im­ men e gravel bars, and giant ripple marks. Th e traveler has reached the starkly sceni " Chan­ neled cab lands," and this d ramatic hange in th e landscape may well ca use him to w onder " w hat happ n d here? " Th e answ er- th e grea te t fl ood documented by man. This publica tion, summari zin g th e equence of geologic events that culminated in th e so-call ed ''Spokan Fl ood," w as prepar d in res ponse to a ge neral int re t in geology and a particul ar interes t in th e o ri gin of th e Scab land o ften ex pre sed by th ose ross in g th e State of W as hington. Fi g. 1.- So il-cove red land sown w ith w hea t is show n on th e ri ght; th e Chan­ n led Sca blands is on th e left. 2 114° IDAHO OREGON 0 20 .40 60 MILES Fig. 2. - Geographic setti ng of the Channeled Scablands,castcrn Washington. Geologic Setting Th e name " Channeled cablands" was first used in the arly 1920's by geologist J Harl en Bretz of the University of Chicago, who made a omprehensive study of th e reg ion and proposed th e idea that the erosional features were th e res ult of a giga ntic flood. Al ­ though Bretz' concept sparked a lively controversy, most geologi ts today agree that the Scablands were carved by a flood of unprece­ dented proportions that too k place 18,000 to 20,000 yea rs ago during the Great Ice Age. The Spokane Flood left it mark along a o urse of more than 550 miles, ex tending from western M ontana to the Pacific Ocean, but the most spectacu lar flood fea tu res were carved into the black volcanic rock terrain in eastern Washington. This rock, the "floor" of the S ablands, isba alt- a dens e crystalline lava that covers more 3 Fig. 3-Vertical dike of basalt cu t­ ting lava flow . The c arc the fis­ sures now filled with so lidified material that once served as con­ duits through which molten rock reached the surface. than 100,000 square miles in parts of Washington, Oregon, and Idaho. The part of the lava fie ld that underlies th e Scab lands in eastern Washington is a sa uce r­ shaped area of about 15,000 sq uare miles almost co mpletely surrounded by mountains and nearly encircled by three rivers - the Co lumbia, the Spokane, and th e Snake (Fi g. 2, pa ge 3). Most of th e lava erupted during the Mioce ne Epoch of th e Tertiary Period- between 30 mil­ lion and 10 million years ago. At times, one flow followed another at short intervals , but at other times, tens of thousa nds anyon cut into layers of ba alt (ea h layer r pr ents a para t flow). 4 Fig. 5.-Step toe Butte. An i land of o lder rock sta nding above the sea of ba alt th at surro unds it. In thi s area th e basa lt is cover cl by a " frosting"' of loess. of y ars intervened b tween th an 100 miles . flows. Eru p ting from long w id e A ro und th e edges of th e fi ures , the mo lte n ro k fl owed lava field a few hills poke up, onto a hill y terrain of old r rocks i land-like, lhro ugh lhe basa lt. - an area th ai probably looked O ne of th e most prom inent o f much like the present-day coun ­ these, Stepto Butte, nca r Co lfax, try no rth of th lava field. Th e Was hington, has giv n it name molten ma terial crys tall ized in to all such fea tu re Geologists th e fi ure to fo rm dikes. In ca ll any island of o lder rock sur­ places, eros ion has xposed ro unded by lava a steptoe. wa rm s o f dikes th at cui o lder M o lten lava occupi sa grea ter ro k . Th se places are beli eved vo lum th an so lid ifi ed lava. As lo be major so ur es of th e lava. fres h lava slowly cool and crys­ Earl y flows filled the va ll eys talli zes, a hexagonal pattern of and subse que nt flows covered hrinkage joints co mmonl y de­ mo l of th e hi gh hills as layer ve lops at ri ght angles to th e cool­ upon laye r of lava eventuall y in g urface. Th ese joi nts brea k formed a so lid sea of basa lt, in up th e lava into ve rti cal co lumns plac s mo re th an 10,000 fee t of rock. M any of th e basa lt fl ows thi ck. Seve ral individ ual flows, in eas tern Washington displ ay w ith thi ckn ess of mo r th an 75 thi s fea tur ca ll d colum nar fee t, hav be n traced for mo re joint ing. 5 Fig. 6.- Columnar Basalt. La rge regu lar co lu mns exposed in a road cut near Spokane, Washington. Where bas a It encroached upon a pond or lake, th e quickl y­ quenched lava fo rm ed ro un ded bl obs called " pi ll ows" instea d of co lumnar joints. In th e wes tern pa rt of th e lava field, a flow of basa lt engulfed a swamp fo rest but, beca use of th e water, th e molten rock fo rm ed pillow s and did not completely consume th e plants . Among th e foss ilized re ­ main s of th ese pl ants are logs of the fa mo us Ginkgo tree, a species th at has survived fo r 250 million yea rs. In 1934 an area of about 6,000 acres nea r th e town of Vantage, W as hingto n, w here th e ves ti ges of thi s ancient swa mp are ex posed, was set as ide as th e Ginkgo Petri fied Fo res t State Park (F ig. 2, page 3 ). At a different time, but less than 50 miles no rtheas t of the si te of the swa mp fo rest, another advancing lava flow encoun te red a shall ow pond co ntain ing the floati ng body of a dead rhinoc­ eros. Agai n, wa ter prevented the m o lten rock from completely 6 consuming the flesh ; instead, pil­ Hill , and Horse Heaven Hills are lows of lava enclosed the body example . Several of these can form in g a crude mold. In 1935, be seen along the west side of the site of the incident was the lava field between Wenat- discovered in the form of a chee and Pas co ( Fig. 2, cavity in a ba alt cliff ( Fig . 2, page 3 ). Of particular interest page 3 ). The cavity contai ned is the Coulee Monocline, an severa l teeth and numerou asymmetric fold that trends pieces of fossi li zed bone. St udies across the north-central part of of the shape of the cavity and the lava field. The physical fea­ the fossil remains showed the tures of this monocline were to rhino to be o ne of an extinct play an important role in deter­ species of Diceratherium . mining the nature of the ero­ After the eruptions ended, sional features of Grand Coulee the lava fie ld was tilted as a unit carved during the great flood. to the southwest. Today the Beginning some time after northeast rim of the saucer-like the flow of lava ended, a cover fie ld is about 2,500 feet above of windblown si lt or loess began sea leve l whereas its lowest to accum ul ate over much of the point, near Pasco, Washington, is lava fie ld, eventually producin g less than 400 fee t above sea level. the fertile soi ls of the Palouse In addition to regional tilting, the country of southeastern Wash ­ lava field was deformed in places ington. The loess attained its to yield a series of fold ridges.