82 BOTANICAL GAZETTE [august 



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 

 i2io m north and south by 405 m 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 10 or i5 m , 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 2o cm 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 cm ) 

 and in the drainage valley (6o cm 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 Britton and Brown's 

 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. Brodie, to whom I am greatly indebted for this courtesy. 



This problem has been carried on under the direction of Dr. Henry 

 C. Cowles, to whom I wish to acknowledge my indebtedness for many 

 valuable suggestions and criticisms during the progress of the investi- 

 gation. 



Geology and topography 



To appreciate fully existing conditions, an epitome of the post- 

 Cretaceous geological development of the region is necessary. The 

 Cretaceous was terminated by that great uplift and crustal movement 

 which formed the Rocky Mountains and gave birth to the major 



