29 



Probably the present local swamps are the vestiges of large continu- 

 ous marshes which formerly extended the whole length of the river, 

 and the hydrophytic plant associations now between the dunes are 

 doubtless the survivors of an earlier swamp vegetation. Slow drain- 

 age has pemiitted the formation of extensive muck deposits, while 

 in the Illinois river valley more rapid and complete dramage has 

 merely left areas of a sandy loam between the dunes. 



The Dixon Area. — A small outlier of this general area, situated 

 four miles (6 km.) southwest of the city of Dixon, is referred to un- 

 der this name in the subsequent pages. This area is not forested, 

 but the small marshes among- the dunes indicate by their vegetation a 

 close similarity to the rest of the area. 



The Oquazvka Area. — Below the mouth of Rock river the Mis- 

 sissippi turns sharply to the south and follows a generally southerly 

 direction for about 60 miles (100 km.). Through this portion of its 

 course, from Muscatine, Iowa, to Ft. Madison, Iowa, its valley is 

 well filled with sand deposits. These are probably chiefly a continu- 

 ation of those along Green river, derived from outw'ash from the 

 Bloomington moraine. It is possible that some of the sand is derived 

 from the Wisconsin river out wash, as described abo\e under the 

 Hanover area. 



At the northern end of this area the principal deposits lie on the 

 Iowa side of the river, where their vegetation has been briefly de- 

 scribed by Pammel {iSgp). In Illinois the sand extends in a strip 

 thr-ough the western part of Mercer and Henderson counties, lying 

 usually close to the river, and gradually becoming thinner and less 

 nearly continuous toward the south. A branch of the Chicago, Bur- 

 lington and Quincy railway ci'osses the deposits between Aledo and 

 New Boston and follows them south from Arpee to the junction 

 with the main line at Gladstone. The town of Oquawka is situated 

 on the deposits, and is a convenient location for the study of the sand 

 vegetation. 



At the north end of Henderson county the sand lies in large, con- 

 tinuous, nearly level areas, with here and there at wide inten^als a 

 low ridge. Its general height is 30-50 feet (10-15 ^'^•) above the 

 river. The ridges rise a few feet higher and near the Mississippi 

 the river dune reaches a maximum height of about 100 feet (30 m.). 

 Toward the south the sand lies in irregular, gently rolling ridges, 

 not more than 30 feet (9 m.) high, and separated by areas of a sandv 

 loam. South of Oquawka the deposits are broken by the Henderson 

 river, but beyond it low ridges reappear and continue to the southern 

 edge of the county. 



