GENESIS OF THE ARKANSAS VALLEY 489 



sediments an^-where else on the American continent, being 

 upward of 20,000 feet, according to Branner.' The peculiarities 

 of this great sequence of soft shales have lately been discussed 

 in some detail, and the real significance of the Arkansan series, 

 as it is called, pointed out.^ 



Thus, independent of whatever geological structure the 

 Arkansas valley may have, the enormous column of shales was 

 of such character as to enable the great stream to scoop out a 

 trough sufficiently vast and broad to give its topographic form 



Fig. 2. — Stratigraphy of the Ozark Highlands. 



the effect of a depression between two uplifts. 



There is another deceptive feature connected with the valley 

 of the Arkansas, that must be taken into consideration. Besides 

 being a topographical trough, the valley is also a structural trough. 

 A broad and shallow syncline stretches from the crest of the 

 Boston Mountains to the first range of the Ouachitas. The strata 

 closely folded in the extreme southern part of the highland dis- 

 trict spread out rapidly towards the north until they form gentle 

 undulations that are so characteristic of other parts of the Mis- 

 sissippi basin. The Ozark arch in Missouri constitutes the last 

 great swell northward. Its southern limb passes into the broad 

 syncline which contains the Arkansas valley. This relationship 

 of structure is represented by a north and south cross section 



(Fig- 2). 



The operation of different geological processes may be either 



■Am. Jour. Sci., (4), Vol. II, p. 235, 1896. 



'Bull. Geol. Soc. America, Vol. XII, p. 173, 1901. 



