Origin of Treeless Plains. 235 
changes is illustrated by a study of the loess deposits. AUGHEY gives the 
following section’) of the loess of Nebraska: 
Bel 4 feet. 
2. Blacksoil EEE RE TRRN 5 
a 5 RE OR 
2 Blacksoll... ...; .., .,. 1-'/,> 
a ET 5 > 
05: DIBORBOSE 2.200,02: 1-"/, > 
7. Stratified loess ....15 > 
It would seem that the loess was deposited that it was covered by a grassy 
sod, which lasted for sometime to be covered again by loess and that the 
black soil layers represent the soil formed by the accumulation of material due to 
the grass vegetation. For deposits of fine grained loess-like beds are forming at 
the present time, wherever dust-laden winds have their velocity checked and 
especially where a carpet of grass or other vegetation covers the surface in 
such places so as to retain the dust that falls upon it 
Origin of Treeless Plains. (Prairie Flora.) ‘The origin of the treeless 
prairies is found in the past physiographic conditions of the region coincident 
with the loess formation and the gradually increasing dry summer climate and 
hot scorching winds, for where the dryness of the summers make the growth 
of trees precarious, the nature of the soil, whether coarse or fine, becomes 
the controlling factor, starting with a grass flora when the central plain was 
ormed. This plant formation was maintained and preserved because through 
the character of the soil, lacustrine at first and subsequently glacial and loess, 
the grasses were enabled to form close mats sufficiently dense to prevent the 
encroachment of tree vegetation. The grass formation represents a type 
of vegetation which is peculiarly adapted edaphically to maintain itself in the 
fine loess soils of the prairie in a region with strong and cutting winds and 
dry climate and to successfully hold its own against other plant formations. 
The experience of orchardists and tree planters in the prairies has demon- 
strated that the eastern trees, on good soil, cannot compete singly with the 
prairie grasses, no matter how great a supply of moisture is furnished either 
by rainfall, or irrigation. There is only one way in which trees of the East 
can overcome the prairie grasses and that is by advancing in solid phalanx. 
This method is necessarily very slow and accounts for the lack of time since 
the glacial epoch for the trees to extend their supremacy over the whole 
prairie region. The buffaloes also had their influence in preserving the prairie 
mat and the Indians hunting the buffaloes set fire to the dry grasses and still 
further increased the prairie tendencies °.. That this historic explanation is the 
correct one is demonstrated by the fact that where the loess is shallow and 
where the glacial gravels protrude there are usually found groves of forest 
trees. The groves of trees which stand isolated upon the prairie grow upon 
1) AUGHEY: Physical Geography and Geography of Nebraska: 276. 
2) See CLOTHIER, G. L.: Forestry and Irrigation IX: 95. Feb. 1903. 
