QUATERNARY FORMATIONS THE DRIFT. 



227 



proach of the sand deposits above and below, toward each other, have 

 forced the conviction that these latter really join at points where the 

 erosion has not taken place, and that the beach ridge that I have 

 already described marks their junction, and that they enclose the red 

 clay deposit as an equitant leaf does its fellow. 



FIG. 14. 



SECTION NORTH OF "THK POINT," NEAR RACINE. 



I. Bowlder Clay. 2. Beach Formation C. 3. Modified Red Clay. 4. Beach Formation D. 5. Sup- 



posed Original Surface. 



It seems clear that these formations were produced by an advance 

 and subsequent retreat of the great lake. The advance was attended 

 by a deposit of sand and gravel along the beach, where the material 

 was acted upon by the breakers, and by a deposit of fine, clayey ma- 

 terial on the undisturbed bottom off shore. This progressive move- 

 ment continued until the position of the beach ridge already men- 

 tioned was reached, where a halt was made for a time, during which 

 the ridge was thrown up by the waves, after which the water retired, 

 but at a higher level than the line along which it advanced, as would 

 be necessitated by the filling up caused by the deposits made in the 

 interval. As the lake retired, the upper beach deposit (D) was formed, 

 and the deposit of clay off shore continued. This explains com- 

 pletely the cuneate form of the clay deposit. Taken as a whole, the 

 clay was synchronous with the beach deposits, though in any given 

 vertical section, it was later than that below, and older than that above. 



This will be made more clear by an examination of the following 

 partially ideal figure, illustrating these formations. 



FIG. 15. 



1. Racine limestone. 2. Bowlder Clay. 3. Beach formation C. 4. Modified Red Clay. 5. Beach 

 formation D. 6. Beach formation in progress. 7. Off-shore clay deposit in progress. 



These formations are isolated from the red clays and beach deposits 

 previously described, and their relations to them are not certainly 

 established, and therefore the provisional names originally assigned 

 them are retained; and in the absence of demonstrative evidence, and 



