it often marks pronounced changes in lithology. If this interface pro- 

 duces a reflection, the thickness of the first layer can be determined 

 directly from seismic reflection profiles. Where the first layer con- 

 sists of suitable sand, available volumes can then be estimated. Where 

 the first layer is unsuitable but overlies suitable deposits, the over- 

 burden thickness can be measured in the same manner. 



A persistent reflector, believed in most places to mark the inter- 

 face between the surficial layer and underlying deposits composed of 

 different material, occurs in eastern Lake Michigan. This reflector 

 (here called the blue vefZeotov) was traced from line to line over a 

 substantial part of the study area, but could not always be followed 

 continuously. Gaps occur either because of poor record quality or in- 

 sufficient acoustic contrast at the interface. The blue reflector is 

 noted on the reduced profiles in Appendix A. 



c. Sediment Characteristics . Brownish-colored quartz sand and light- 

 gray silty clay are the dominant sediment lithologies found in cores from 

 the study area (see Table). Gravel, sandy gravel, till, and shale occur 

 locally. Where sampled the surficial sediments above the blue reflector 

 consist of quartz sand which is fairly uniform in composition throughout 

 the area. 



Core data are not adequate to determine the distribution and charac- 

 ter of deposits below the blue reflector. Available data suggest that 

 these sediments probably consist chiefly of clay and silt in the area 

 between Manistee and Pentwater, in the northern part of the area, and 

 from Saugatuck to Burns Harbor in the southern part. Between Pentwater 

 and Saugatuck, sand apparently underlies the blue reflector in many, if 

 not most places; however, gravel and till-like deposits of silt, sand, 

 and gravel occur in a few places between Saugatuck and Benton Harbor. 



Sand from the study area is typically a light-brownish color (Munsell 

 Soil Color Code 10 yr 5/3 to 10 yr 7/3) (Munsell Soil Color Charts, 1954 

 ed. , Munsell Color Co., Inc., Baltimore, Md.). It is fine to coarse in 

 size (Wentworth Scale) and contains little or no silt or clay. Micro- 

 scopic examination of a few representative samples indicates that 80 

 to more than 90 percent of the grains are quartz with the remainder 

 principally black opaques, and pale-yellow or reddish-colored trans- 

 lucent grains with very small amounts of calcareous material consisting 

 of mollusk shell fragments and ostracod carapaces. Nearly all the surfi- 

 cial sands from the study area appear to be uniform in general character 

 and composition. 



Silty clays occur predominantly below a surficial sand cover, and 

 are mostly light gray (N7) or light brownish gray (10 yr 6/1) in color 

 with a few pinkish-gray (5 yr 6/2) occurrences in the northern part of 

 the study area. The silty clay is plastic when wet and contains varying 

 amounts of sand. The silt-clay ratio and bulk engineering properties 

 have not been determined. 



