34 '/'/;<• .IlllCliitUl GCdlo^i.Kt. .iMnmuy, IIM.I. 
(Diiiitiy el llif l\iiij;'s livci" Iiydro^r.ipliic l>;isiii in Aikansas, 
our iii.iy slaiitl i>ii one of (he ridj^cs of llie ICuicka iiKMtntains 
ami look lar to llic eastward across a vast complfx of hills. 
Three base-levels arc distiiutly notiii ;d»lr. Tlir snininits of 
many of ihe ridj^es form llu- dissrcled floors of basin valleys 
often seviMal miles in width, and which occupy three-fourths 
ol the entire sni faei-. 'This is the Lafayette base-level. Sev- 
eral Imiidred fitl hiidiei. the main ui)lan(l ridges represent the 
'IVrtiary penepl.iin. I.ookinj; across their summits the very 
hilly eomUry in the far dislancc apparently nicrpfes into a 
plain. The sky-lim- is even with llu> exception that a few 
monadJiocks rise al)o\e the penepl.iin. .Some of Ihem are 
cone-shaped, htit sevi'i.il are elongated into ridj^es whose crcsts 
are iven and snmmits Hat. su^gvstiuR* the Cretaceous base- 
level. In all the O/ark ii\i;ion, this is one of the most instruc- 
tive to the student of physio{;ra])hy. 
The existence of the basin valleys, rock-terraces, and de- 
pressed areas anunic^ the hills of the entire Ozark hif;hland 
conntry is a fad which m.iy l)e verifuHl by anyone who doubts 
it. It is .also ;i fad that they Iiavi' their levi-l irrespective of 
tin- rocky structure, and ih.it the fri-cpienl concurrence of the 
lloor of the basins with the to]) of centain formal it)ns is tucrcly 
foitnitous. These basin lloius .ind rock leri'act's olten bevel 
the edt^es of the strata, and they m;iy be observed to pass from 
one foinialion to another witlu>nl deformation, lleuce, there 
is I'very re;»son to ct>nsidi'i- them to represent a base-level of 
erosion distinct fi'om and considerably later than the main 
Tertiary peneplain. In many i)laces fnMU the Arkansas to the 
Missouri rivei-, this base-level is the site of renuiants of an 
.mcietU river alluvium containinj;" a peculiar brown {.gravel 
which is known to bo of Lafayette age. The presence of this 
I ivcr-gravel is corroborative of the Lafayette age of the base- 
le\el ;is drawn fnMu purely j^vomompholoqic evidence. 
The vicinity of the White river appears to be the region 
in which there is the greatest interval between the main Ter- 
tiary and the Lafayette baso-lovels, here reaching a probable 
maximum o{ 300 feet northward from the "crest of the 
Ozarks;" in the Osage basin and along the Missouri river, the 
ditTcrence in level is only about seventy-five feet. In this re- 
i;ion the duplex char.acter of the valleys is not so prominent. 
i 
