66 The American Geologist. February, i904. 
S. E. of the axis occupies only about 95 to icxd square miles, 
rising within three miles to a hight of about 65 feet above the 
bottom of the trough. The latter as a whole is bounded on the 
N. W. by a massive ridge which begins somewhat abruptly 
near the northern line of Iowa, then follows the "great bend" 
of the Big Sioux near Sioux Falls and maintains a more or 
less westerly course along the northern line of Lincoln county, 
until S. W. of Lake City, beyond the limits of the area, it 
sweeps around the west and N. W. continuing in that direction 
past Montrose in McCook county and Winfred, Lake county, 
for about 60 miles, up to Vermillion Point. 
It has undergone not half as much erosion as its comple- 
ment bordering the trough in the S. E. which vividly reminds 
one of a mighty plasmodium stretching out its tentacular 
pseudopodia in all directions, but mainly toward the south and 
east. This ridge begins its course about 2 miles south of Can- 
ton and trends for the first 6 miles in a S. W. direction, when 
it runs almost due west for about 7 miles. At that point it 
seems to end, rapidly subsiding to the bottom of the trough 
at an approximate rate of 80 feet per mile. A narrow spur, 
however, which projects about 5 miles east of that point from 
the southern side of the ridge for a distance of 4 miles, as well 
as some detached portions farther S. W. continue the ridge in 
an average southern direction through Beresford, beyond the 
southern limit of the region. 
Both ridges rise to about the same relative hight above the 
axis of the trough, i.e. 180 feet, while in their highest eleva- 
tions they are from 1 560-1620 feet above sea level. Both serve 
as watersheds for the streams within as well as for those with- 
out the trough. 
The largest streams of the area run almost parallel to each 
other at a distance of about 20 to 25 miles and may be taken as 
natural boundaries ; these are the Big Sioux to the east and the 
Vermillion river to the west. 
From the western watershed descend into the trough Nine 
Mile creek and Beaver creek which empty into the Big Sioux 
and Long creek which is an eastern tributary of the Vermil- 
lion river. But while Nine Mile creek takes its rise far below 
the foot of the ridge a little E. S. E. of Tea, deriving its waters 
apparently from a sand stratum which extends farther north 
