24 BOTANICAL GAZETTE [july 



lee slope just where the edges of the northern glacial ridge disappear 

 under the level surface of the bar. 



The glacial ridges consist of surface deposits of sand and gravel 

 more or less water washed and stratified, but contain below at 

 least one layer of laminated clay several feet in thickness. This 

 layer appears on the Michigan shore bluffs and is occasionally 

 exposed by erosion on hillsides and in ravines. On the shore end of 

 the southern ridge is located a second small group of dunes about half 

 a mile north of Frankfort. They are half a mile in length and one- 

 fourth of a mile in width, and extend almost directly north from 

 the shore, which at this point lies northwest and southeast. The 

 group consists of small fixed dunes about 50 ft. in height, and the 

 ridge is itself about 100 ft. above Lake Michigan. These fixed 

 dunes have been blown out through the center in a long trough, 

 which ends in a large, steep-sided, semicircular blowout popularly 

 called the " Crater." For 100 yards or so from the edge of the shore 

 bluff the sand has been blown away to or below the level of the 

 glacial deposits, which are exposed in the bottom of the trough. 



CHARACTER OF ENVIRONMENTAL FACTORS 



Climatological. — So far it has been possible to obtain only 

 incomplete and not entirely satisfactory observations, so that 

 only a brief general statement will be given. There is nothing 

 exceptional about the meteorological conditions of this region as to 

 temperature, precipitation, or moisture in the air and in the soil. 

 On account of the marked projection of Point Betsie into Lake 

 Michigan, it is exposed both to southwesterly and northwesterly 

 winds, which probably accounts for the large amount of moving 

 sand around the Point. The wind also has an indirect influence 

 on evaporation and temperature, especially in summer, as a marked 

 difference in both is observed when a period of easterly winds is 

 followed by a similar period of westerly winds. 



SuBSTRATUM.^ — Open dunes. — On the dunes the blown sand is 

 generally homogeneous in physical character, but a marked char 7 

 acteristic is the large percentage of calcium carbonate present in 

 the form of residual grains formed by the grinding up of shells, 

 apparently chiefly of gastropods. This calcium carbonate content 



