292 



RELIC DUNES* 



Dunes form one of the typical stages in the construction of beaches 

 and they may also be one of the stages in the destruction of a vege- 

 tated beach, when they may be termed "relic dunes." (See group of 

 dunes, PI. XLVI, Fig. i.) The vegetation north of Winthrop Har- 

 bor is bordered on the lakeward side by a low ridge which supports a 

 very dense growth of J uncus balticus lit to rails. When the lake begins 

 to cut into the beach it washes away sand from the Juncus, leaving an 

 exposed bluff of densely intertangled roots. In weak spots the waves 

 are able to wash their way entirely through the ridge of Juncus to 

 the grassy plain beyond, which is easily destroyed as far as the waves 

 have power. In places the Juncus is left as a mound with its sides 

 perpendicular and densely coated with exposed roots. This is an early 

 stage of a relic dune. (For such dune, shown in detail, see PI. XLVI, 

 Fig. 2.) As wave action continues, the onwash and the backwash of 

 the waves, in combination with the wind, reduces the dune from the 

 appearance of "A" (Fig. i, PI. XLVI) to that of "C," in which the 

 sides are sloping. These summer secondary stages look very much 

 like ordinary dunes except that they are more or less coated with ex- 

 posed roots. In course of time the dune is entirely washed away. 

 During winter the disruptive power of freezing water is an important 

 agent in the breaking up of the dunes. The effect of a severe frost 

 immediately following a heavy rain upon one of these dunes is shown 

 in Figure i, Plate XLVII. 



These dunes are prominent features of the vegetation of the beach 

 from the state line to Kenosha. With the Juncus are associated a few 

 plants of relatively little importance, such as Sporoholus cryptandrus, 

 Russian thistle (Salsola kali tcnuifolia) and dogwood {Cornus sto- 

 lomfcra). Besides the Juncus relic dunes, there is also a single ex- 

 ample of a relic dune formed by Jnnipcrus couiuiunis deprcssa (see 

 "D," Fig. I, PI. XLVI). Its sides are not so steep as those of the 

 Juncus, and most of the ^'egetation is on the lakeward side. The sand 

 that accumulates somewhat in the rear of the dune is not washed away 

 rapidly because the dune is so near the limit of wave power. During 

 the course of the next few decades there w'ill be eight or ten of these 

 Jnnipcrus relic dunes, formed by both Jnnipcrus conununis deprcssa 

 and J. horizontalis. 



THE MAN-MADE DUNE 



In order to protect the golf grounds at the southern edge of Ke- 

 nosha from blowing sand, a long dune about two meters high has 



*See p. 277. 



