25 



long been well known, and are very characteristic of the rest- 

 less energy of that immense river. Near the south end of the 

 river dune just described, the river is even now; operating to 

 destroy the product of many years of plant activity. Below the 

 islands already mentioned the current sets across from the Iowa 

 side, and striking the base of this river dune veers off to the 

 south along the Illinois shore. The current has broken down 

 the barrier of leaf-mold and intertangled roots and is rapidly 

 eroding the sand. This erosion begins at the base of the dune, 

 exposing the bare sand. As more and more of the sand is 

 carried away, the upper portion of the dune is undermined and 

 begins to settle down toward the river. It is now seen that 

 the principal mass of roots has extended only one to one and 

 a half feet below the surface, forming a coherent stratum rest- 

 ing on the loose sand beneath. The loose sand rests at as 

 steep an angle as possible, and irregular detached blocks of the 

 coherent surface layer slide slowly down the incline toward 

 the river. Their motion is of course very slow, and partially 

 dependent upon the rate of erosion. But that they are loose 

 is at once demonstrated by stepping on one, which then im- 

 mediately starts down and in ten seconds to a minute, depend- 

 ing upon the distance, comes to rest on the flat beach at the base 

 of the dune. That the plant population is a relic of the former 

 mesophytic conditions is shown by the species, which are largely 

 perennials found also on the mesophytic slope. Prominent 

 among them are bush clover, spider wort, Tradcscontia rcflcxa 

 Raf., horsemint, goldenrod, Solidago nemoraUs Ait., and worm- 

 wood, Artemisia candata Michx. Naturally such species as the 

 anemone or the fern could not be expected to persist under 

 such conditions. The flora of the sliding- masses stands in sharp 

 contrast with the meager vegetation of the general slope, com- 

 posed mainly of partridge-pea and a few other annuals. These 

 plants constitute the blowsand association, identical with the 

 first association of the river dune. 



At the top of this interesting slope the trees are being un- 

 dermined also, and such a mesophytic species as river birch ap- 



