372 PLEISTOCENE OP INDIANA AND MICHIGAN. 



siderable size in the district of these modified beaches and the remarkable way in which the 

 gravel taken from them was swept up the slope and built into the Whittlesey beach is a fine illus- 

 tration of the general principle that surf, beating upon an offshore shallow bottom, tends to 

 loosen coarse material and carry it up the slope. Sometimes currents along the shore domineer 

 over the tendency to on-shore deposition and the coarse material in such cases is carried along to 

 some more favorable place of deposit. The Arkona ridges were swept away down to their roots 

 or basal parts which by that time presented so little relief upon the lake bottom as to give the 

 oscillating waters no effective hold at any particular point. It was mainly after this condition 

 had been attained and during times of quiet that the clay was deposited in the gravel. 



It was probably in some such way as this that the Arkona beaches were transformed from 

 the robust type seen in the Black River valley to the present low, clayey, gravelly belts. 



DISTRIBUTION. 



As far south as Lenox the three Arkona gravel belts are distinct. Beyond, there are a few 

 intervals in which only two are discernible. Still farther southwest the first and second ridges 

 draw closer together and the vertical interval between them diminishes until it is often difficult 

 to distinguish the two. The three ridges are distinguishable some distance beyond Britton, 

 where the upper two are of very nearly the same height. 



The same occurrence of three beaches was noted in the Saginaw basin between Flushing and 

 Cass City, but west of Flushing only two beaches appear. Those observed in the Saginaw basin 

 resemble very closely in their relations, though not in their strength nor in their modifications, 

 those running southwest from Spring Hill. In the Saginaw basin the Arkona beaches were not 

 submerged, but they were uplifted. Nothing, however, like beach A in the Black River valley 

 was observed. The full strength of the Arkona beach ridges in the northern parts of the two 

 valleys — near Cass City and near Croswell — seems to make it clear that the ice barrier at the 

 time of Lake Arkona stood at least 25 miles back — far enough away to allow of a heavy surf on 

 shore, apparently as heavy at Cass City and Croswell as at other places farther south. 



SUBMERGED DELTAS. 



The low gravelly belts which represent the beaches of Lake Arkona are splendidly supple- 

 mented by the deltas that were built in this lake. Not only are these deltas strongly developed 

 and prominent as sedimentary deposits, but they are modified in precisely the same way as the 

 beach ridges. 



The formations here described as deltas in Lake Arkona were not at first recognized as such, 

 but were explained as subaqueous extensions of deltas of Lake Whittlesey. Their unusual 

 characters, however, were not satisfactorily explained in this way, and their true nature was 

 recognized only when the modified beaches of Lake Arkona had been traced and their peculiarities 

 explained. It then became apparent that the deltas lay at the same general level as the modified 

 Arkona beaches and that their peculiarities were due to submergence. 



The gravel belts representing the beaches are generally somewhat expanded wherever they 

 cross stream courses of even moderate size. Instances occur on Mill Creek southeast of Avoca, 

 and a distinct delta deposit appears to be connected with the lower Arkona beach a mile or so 

 northwest of Abbottsford station. The best examples, however, of Arkona deltas were found 

 farther south in connection with some of the larger streams, most notably on Clinton River above 

 Utica, on Rouge River below Plymouth, on Huron River below Ypsilanti, on Saline River at 

 Milan, and on Raisin River near Blissfield. The best of all is the delta of the Huron southeast 

 of Ypsilanti, and this and the delta at Milan were studied in some detail. 



The delta of the Huron is a plain 4 or 5 miles wide with an almost semicircular southeast 

 front. This delta, and in fact all of the Arkona deltas, protrude farther into the lake bed than 

 do the deltas of any other stage of the lake waters. This relation of the Huron delta in Lake 

 Arkona is well shown on the map in the Ann Arbor f oho. 



