l^ ro^OfA ACADEMY OV SCrESi'CF.S- 



The tT(fiii;!i of t!i(' \[iss()uri has Ix'cu funned sirrce the 

 fiflveiit of t!i(' Ico Auc I>;ik(' ('hcvcnuc liua-ered till the 

 svatcrs from the i^rcat f •c-sIj 'ct reph'iiislicd it. AI)()V(' Yank- 

 ton t!u' Missonri mwcs its |)i-cs<'nt cunrsc. and poi^sihly it.S" 

 exi&tence^ to the iitHuciuc aud in^t "rf(n-(>nc(^ of the ice-sheet. 

 The disti-jhution of ])re-i;lacial hieustrine (h'posits, and the 

 'liift and h)'.-ss, the character and iM-iitht of t!ie terraces the 

 I ontinucd coiTasifMi of the hottoiu of the trongh tu the pre<- 

 «'nt time, a U sustain these conclusions. 



THE ORIGIN OF THE EXTRA-MORAINIC TILL. 



V.V I'koK. j. K. 'lODl). 



Tliis [MjK^r ti-eit;'d pai'ticnhirly of th ■ nsoraiisi" tifl of th<" 

 Missouri \ alley. To explain th • orJLiin of tliis till, tii-rc are 

 two theories acceptal)le ; one tliat it is suU-ulaci d, the oth 'r, 

 that it \va?* formed at the bottom of a lil icial lake, i>radiially 

 Hilino- with dehris hrou-iit from a iilacier and lln^ sliore, hy 

 ice-rafts. 



'Ihe suh-o-hicial lhcor\- is fi;\oie(l l,y (1) its ch se ies(ni- 

 itlance to till unipiest ionahly suh-iiiacia I. ( "2 ) striae on the 

 underlyinii' rocks, { ;> ) the i)resence of osar-like ricjo-cs. (4) 

 its difference from th<' reddish l)oaldery clay, often found 

 a])o\-e it, which is cei-tainly of suI)-a(pu'ous ori<>-in. and ( ."> ) 

 the thickeninii', and ^renter elevation of this till, near its 

 outer mariiin, as alono- the east side of tlu* Vno- Blue in 

 Nebraska, suir.iZ'Cf^tini'- an imi)erfect moraine. 



Aii-ainst these points, respec ti\-ely, it was ui-i>-ed : (1) 

 ( 'lose exannnation, not int're(|uently, re\('als traces of strati- 

 ticatiori, )»y differences of coloi', and in the distribution of 

 pebbles and l)o\il(lers. (2) Stri.e are the exception, e\('n 

 on siiinlarly piominent surfaces. The}', in some cases, show 

 on the same surf.ace directions diNcruiuu- (iO de«>rees, and 



