416 PLEISTOCENE OF INDIANA AND MICHIGAN. 



it first becomes distinct the upper group appears to cover a vertical interval of 25 or 30 feet. 

 Farther north the vertical range of the upper group gradually spreads, until at Mackinac Island 

 it is 47 to 50 feet. 



Below the lowest beach of the upper group there is an interval of 25 or 30 feet apparently 

 without beaches, below which he the three or four beaches of the second or Battlefield group. 

 These beaches, which are not so strong as the higher ones, have been distinguished for a con- 

 siderable distance down both sides of the peninsula from the north but have not been traced to 

 the hinge line. 



Below the Battlefield beaches lies the third or Fort Brady group, consisting generally of five 

 or six rather light ridges, as represented at St. Ignace and Mackinac Island. They he next 

 above the Nipissing beach and are closely set at short vertical intervals. They are entirely 

 distinct from the Nipissing beach below and the Battlefield beaches above. 



North of the Straits of Mackinac the three groups are fairly distinct wherever they are well 

 displayed. South of the straits, however, there is more difficulty in separating them. The 

 principal and most characteristic individuals of each group are generally easy to distinguish, but 

 in some places the intervals between the groups are filled with faint closely set ridges whose rela- 

 tions to the groups can not be determined. 



UPPER OB MAIN GROUP OF BEACHES. 



DISTRIBUTION. 



Port Huron to Port Austin. — From Port Huron to Pointe aux Barques, at the northern end of 

 the "thumb," the Algonquin beach lies generally less than 1 mile and nowhere more than 2 miles 

 back from the present shore. In two considerable stretches it is entirely cut away. Whether it 

 takes the form of a beach ridge, or a spit, or a wave-cut bench and lake cliff, it is always strong. 



The beach, as shown in figure 12 (p. 483), enters Port Huron as a large spit or barrier ridge 

 projecting south along the shore from old lake cliffs at the north and ending near Elmwood Street 

 and Gratiot Avenue. The last half mile of the spit runs southwest to the end, the main part to 

 the north trending a little east of south. It is a gravelly, sandy beach ridge or close-set series of 

 ridges, in many places obscured by fine sand and dunes. Its last hah mile after it curves to the 

 southwest is made up of three or four strong gravelly, sandy ridges. 10 to 15 rods wide, separated 

 by narrow troughs in some places 6 or 7 feet deep. McNeils Creek cuts through the ridges about 

 a quarter of a mile north of the south end of the spit and runs south nearly parallel with St. Clair 

 River for a quarter of a mile before entering it. It is possible that the spit once extended farther 

 south and has been cut away, but no certain evidence of such a change was found. The spit 

 was built from the north, mainly out of material derived from the cutting of the old shore north 

 of Stevens Landing. 



North of the city the ridges pass through Lakeside Cemetery, where they are buried in dunes, 

 some of which are 20 feet or more in height. The cemetery is mainly on these dunes. Except 

 near Port Huron and in less degree between Lakeport and Stevens Landing the gravel ridges 

 of the Algonquin beach are largely hidden by fine sand; and the ridges themselves are com- 

 posed in some places mainly of sand with only a small quantity of gravel. 



From Port Huron north to Lakeport the main ridge curves gently north, being close to the 

 lake near Port Huron, but farther north drawing back to about a mile. The gravel ridges and 

 the strip between the main ridge and the lake are almost entirely covered with dunes and fine 

 sand. From Lakeport to Stevens Landing, about 6 miles farther north, the main ridge runs 

 generally less than a quarter of a mile back from the present shore and is less duney than it is 

 farther south. The land back of it is flat clay, in places 4 or 5 feet below the crest of the ridge, 

 and is rather poorly drained though not distinctly swampy. 



From Port Huron to Lakeport the Algonquin ridges rest on a flat clay floor, which slopes 

 very gently toward the lake and which, behind or west of the main ridge, forms a swampy lagoon 

 6 or 7 miles long. The swamp reaches its maximum width of 1 mile opposite Gardendale, where 

 it is floored with peat for 2 or 3 miles north and southeast. The swamp floor is here 6 to 8 feet 



