82 BOTANICAL GAZETTE {aucust 
in the contiguous portions of Iowa, Nebraska, and South Dakota, In 
the fall of 1904, Yankton, South Dakota, became the base station, © 
and for the next three seasons (1904-1907) the work was carried on 
from that point. During this period the greater part of South 
Dakota east of the Missouri was visited, and repeated trips were made 
into contiguous Nebraska and into Iowa, while at the same time 
detailed study of a restricted area was being prosecuted. This local 
area upon which study has been focused lies in the township of 
Yankton and embraces about 49 hectares, being a rectangular strip 
1210™ north and south by 405™ east and west. The topography is 
uneven and comprises a series of prairie knolls and slopes, separated 
into two groups by a gentle drainage valley which traverses it 
in a southwest and northeast direction. The knolls are low and 
their slopes gentle, scarcely higher than 1o or 15™, with a gradient 
never exceeding 30 or 40°. The entire area is largely underlaid by 
glacial gravel and clayey till. The former mainly constitutes the 
knolls—the humus is here the most shallow, averaging only about 
15 to 20°" in depth. Off from the knolls on the level the humus caps 
the deposit of clayey till. On the lower erosion slopes (25 to 30™) 
and in the drainage valley (60°™ plus) the humus has accumulated 
to a greater depth, sufficient in the latter to bury the till beyond the 
zone of root activity. The humus is in all cases till or gravel modified 
by atmospheric, organic, and biotic agencies. 
To insure uniformity the nomenclature of Brrrton and BRowNs 
Illustrated flora has been followed, except where it has conflicted 
with the verification of grasses made for me through the United 
States Department of Agriculture by Mr. Percy L. RICKER and 
Mr. D. A. Broptz, to whom I am greatly indebted for this courtesy: 
This problem has been carried on under the direction of Dr. . 
C. Cow Es, to whom I wish to acknowledge my indebtedness for many 
valuable suggestions and criticisms during the progress of the investe 
gation. 
Geology and topography 
_ To appreciate fully existing conditions, an epitome of the post: 
Cretaceous geological development of the region is necessary. ~ 
Cretaceous was terminated by that great uplift and crustal movement 
which formed the Rocky Mountains and gave birth to the ne 
