30 The American Geologist. lanuary. looi. 
like monadnocks and catoctins, and beneath its plane are 
trenched narrow basins and canyon valleys of systems to be de- 
scribed later. 
This Tertiary peneplain emerges from among the moun- 
tains to form the very even plain of the Arkansas valley where 
for a width of fifty miles or riiore it is not interrupted by any 
monadnocks. Standing on a slight elevation and looking 
across the Arkansas valley, the surface appears to be a re- 
markably even plain, sloping very gently from the prominent 
mountains on the south toward the Arkansas river, quite per- 
ceptibly eastward or down the valley in Indian Territory w^est 
of Fort Smith, and very decidedly from the Bosioa mountain 
to the river. In short, the deformation of the plaia can be 
very clearly seen from any point of vantage. Along the axis 
of the trough flows the Arkansas river and in its vicinity at 
Fort Smith and Van Buren the Tertiary peneplain has no 
greater altitude than 600 feet above the sea. 
I have said the Arkansas valley appears like a very even 
plain, but in reality it is not. The original plain has been 
pretty thoroughly dissected, and remains only in narrow 
ridges. South of the river, there are the long, straight, even- 
crested east-west ridges as in the inter-montane basins. 
Many of these ridges are 200 to 300 feet in hight and are 
locally known as mountains. In places they are separated by 
considerable basins, and the streams cut through them in nar- 
row gorges like the water-gaps of Pennsylvania. Indeed, the 
topography is that of the eastern Pennsylvania and northern 
New Jersey mountain country on a smaller scale. 
North of the Arkansas river, the ridges which form the 
dissected Tertiary peneplain are less regular in crest-line and 
trend prevailingly in a north-south direction. Now an ex- 
tremely curious feature of Ozark highland structure comes to 
light. Generally the slopes of the peneplains in the Ozark re- 
gion are at a very low angle. But here on the southern slope 
of the Boston mountain the Tertiary peneplain descends 
steeply from an altitude of about 1,700 feet A. T. near Wins- 
low to 600 feet A. T. near Van Buren, a distance in an air- 
line of little more than twenty miles. This gives the pene- 
plain's remnant-ridges on the north of the Arkansas river a 
very decided slope lengthwise or along their axes. In fact. 
