﻿1903] UPLAND PLANT SOCIETIES " 37 



The range of altitude over the whole county is less than 

 122°", so that absolute altitude itself, with its concomitant varia- 

 tions in climatic conditions, is not a factor in the distribution 

 of the flora. Differences in relative level, however, produce 

 marked variations in the drainage, and hence in the water con- 

 tent of the soils. 



2. Geology of the county. 3 — The bed rock of the area is 

 almost entirely covered to a great depth by glacial drift, only a 

 few small outcrops being found within its limits. The drift is so 

 deep throughout the county that the underlying rock layers have 

 apparently no influence upon the vegetation. But the drift 

 itself presents some very interesting features.-^ The land con- 

 sists, in general, of two great blocks of till upland lying on 

 either side of a much lower gravel and sand plain of varying 

 width, which extends in an irregular line from about the middle 

 of the northern boundary southward through Cedar Springs, 

 Rockford, Plainfield, Grand Rapids, Fisher, Carlisle, and Ross, 

 and cutting the southern boundary about 4.8^"" east of the 

 southwest angle of the county. This plain traces the path of the 

 outflowing water as the Michigan ice sheet retreated northward 

 at the end of the last glacial epoch. An indentation or embay- 

 "ment in the southern ice margin during this retreat marked 

 the junction of the two lobes of the glacier, the eastern lobe 

 coming from the region of Saginaw Bay, the western from 

 that of Lake Michigan. It was naturally into this embayment 

 that much of the water was discharged during the melting 

 process, and the line of sandy plains just traced marks, from 

 Carlisle northward, the path of this gradually increasing notch 

 in the edge of the ice. Southward of Carlisle the Green Lake 

 sand and gravel plain (in Allegan and Barry counties) originated 

 m the same wa3\ 



Three well-marked terminal moraines lie partly within the 



^For a more detailed account of these features, see the author's previous publica- 

 tion on the same subject, already cited. A brief description of these features, by Dr. 

 A. C. Lane, will be found in the introduction to Miss E. J. Cole's Grand Rapids 

 Flora, Grand Rapids. 1 901. 



*For aid in interpreting the glacial topography I am indebted to Mr. Frank 

 Leverett, of the U. S. Geological Survey. 



