DESCRIPTION OF THE ENVIRONMENTAL CONDITIONS AND 



DISCUSSION OF THE GEOGRAPHIC RELATIONS 



OF THE BIOTA. 



BY ALEXANDER G. RUTHVEN. 



LOCATION AND DESCRIPTIOX OF THE SAND DUNE REGION. 



Topography. 



The region investigated by this expedition comprises the greater 

 part of the strip of sand ridges that extends along the south shore of 

 Saginaw Bay, from Hardwood Point to Sand Point, "in fact at intervals 

 all along the western shore" (Da\'is, 1900, p. 40). This sand area 

 varies from about three-fourths to one and one-half miles in width 

 and consists of a succession of sand and gravel ridges (in general parallel 

 to the shore) with intervening swales and swamps. Beginning, accord- 

 ing to Davis (1900. p. 56), "at the north extremity of the county 

 around Point Aux Barciues," there is a continuous ridge of sand hills, 

 "which continues swinging from point to point, like draped festoons, 

 clear to the southwest edge of the county. Exceptionally high knobs 

 rise more than 40 feet above lake (620 A. T.). but in general the top 



of the ridge is a little above 30 feet above lake level On the 



landward side this ridge is backed by extensive swamps, now rapidly 

 being artificially drained, at an altitude of 20 to 24 feet above lake 

 (606 feet A. T.). On the lake side we find series of sand and gravel 

 beach ridges, interspersed with swamps. Their altitudes are quite 

 various, depending somewhat upon how far the ridges pass into dune 

 sand aggregates. There are in places at least four or five strand lines." 



^lany of the ridges are low and composed largely of gravel, being 

 very evidently old beaches. Others consist of small dunes superim- 

 posed upon the old beaches, and two are high ridges of pure sand that 

 are unmistakably fossil dunes. Even in the case of the latter, however, 

 the dunes have been blown up on an old beach and have undergone 

 little subsecjuent migration. This is well shown where blow-outs cut 

 them to the base, revealing the underlying beach. (Plate III b.)/ 

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