44 IOWA ACADEMY OF SCIENCES. 



the forest and every shower seams and scars the denuded 

 hillsides. 



But plants assist not only in preventing the tearing 

 down of such deposits, but they often assist in building 

 them up. That they serve as holdfasts for material car- 

 ried by water is shown on every lake shore and along every 

 stream in the state. Plate IX illustrates such a beach on 

 the shore of Spirit lake. As the lake subsided in recent 

 years the water receded, exposing the beach shown in the 

 figure. Soon sedges and other beach plants appeared, and 

 each wave that swept over them during storms, deposited 

 more material, and the beaches were gradually elevated. 

 Other plants, such as Artemisia, etc., then took posses- 

 sion of the higher, now dry beaches, and made possible 

 the accumulation of a new soil from the materials washed 

 down from higher grounds, or blown up from the beach. 

 As this new soil was formed the more luxuriant vegetation 

 of the higher shores gradually crept out upon the sandy 

 beaches, until to-day such vegetation grows in abundance 

 in places' which within the memory of the writer were 

 perennially covered with w^ater. 



Plate X, representing a low alluvial bar covered with 

 vegetation, furnishes another illustration. Thirty years 

 ago the main current of the Iowa river swept over the site 

 of this bar. As the current shifted to the east a bar of 

 sand and gravel was gradually formed on the west side. 

 Only a few^ years ago this bar was almost barren of veg- 

 etation. Then sand-loving sedges and grasses appeared, 

 and other plants — ^weeds especially — gradually took pos- 

 session of the higher parts. The bar was periodically 

 flooded, and each receding flood left more or less alluvium 

 among the lowly plants on the bar. Then the sand-bar 

 willow established itself, the accumulation of alluvium 

 became more rapid, and that part of the bar became higher 

 after each overflow, and finally only the higher floods were 

 able to reach it. Then seeds of other trees were carried to 

 it by wind and water, and the black willow, almond-leaved 

 willow, black birch, white ash and cottonwood gained foot- 

 hold. The lower part of the sand bar, next to the river, 



