MICHIGAN ACADKMY OF SCIKNCK. 201 



southeastward from sand bluffs on the lake. On either side of them the 

 sand has been blown out by both southwesterly and northwesterly winds, 

 and a semicircular trough with a very large horseshoe front has been 

 formed. This is one of the largest blowouts in the whole dune region 

 of Lake Michigan, and is unique in this locality at least in showing the 

 influence of winds from two directions. Beyond this blowout the dunes 

 are higher and the steep sand cliffs begin again, and finally end about a 

 mile north of Point Betsie with a definite lee slope just where the edges 

 of the moraine ridge grade into the level surface of the bar. Under these 

 dunes the moraine apparently extends some distance to the south as 

 glacial pebbles have been found in situ almost to the top of the bluffs on 

 Lake Michigan to a point within a quarter of a mile of the lighthouse. 



b. The Crater Group. (Plate VI, Fig. 2.) This second group of 

 dunes is found half a mile north of F'rankfort on the shore, forming a 

 most interesting group of detached, perched dunes. They are only half 

 a mile in length and one-quarter of a mile in width, and extend almost 

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

 The group consists of small fixed dunes which evidently once extended 

 much farther out into the lake. They are from 50 to 100 feet in height, 

 but are placed on a morainic plateau, which itself rises 100 feet above 

 the lake. These fixed dunes have been blown out through the center in 

 a long trough, which is itself complex and shows traces of a number of 

 parallel blowouts. These all end in a large steep-sided, semicircular 

 blowout, popularly called the "crater." 



II. CHARACTER OF ENVIRONMENTAL FACTORS. 



A. Climatological. So far it has been possible to obtain only in- 

 complete 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 precipitation and 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, as well as for the large blowout described 

 above. The wind also has an indirect influence on evaporation and 

 temperature, especially in the summer, as a marked difference in both is 

 observed when a period of easterly winds is followed by a similar period 

 of westerly winds. 



B. Substratum. On the dunes the blown sand is generally homo- 

 geneous in physical character, but a marked characteristic 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. 



