808 APPENDIX. 



The bed of tbe river opposite the city has been stated to con- 

 sist of limestone rock, but without any proof or much probabiHty, 

 From the fact of its affording a good anchorage to vessels, I am 

 inclined to think that it is wholly composed of clay and gravel. 



Detroit Fluviatile Clay, — The argillaceous stratum of De- 

 troit extends along both banks of the river to its head ; passes 

 around the shores of Lake St, Clair, and up the Eiver St, Clair 

 to Fort Gratiot — a distance of seventy miles. In this distance 

 there are some moderate elevations and depressions in the sur- 

 faces of the soil, but no very striking changes in its general cha- 

 racter and composition. The boulder stratum is prominent at 

 Gros Point, at the foot of Lake St. Clair, where the shore exhi- 

 bited some heavy blocks of granite, and other foreign rock, 



St, Clair Flats of Plastic Clay. — At the mouth of the River 

 St. Clair, the current is divided into several channels, and spread 

 over a considerable tract of low ground, which is covered with 

 grasses and aquatic plants. These channels ^ave worn their way 

 through beds of tough blue clay, called the flats, over which 

 there is sometimes not over seven feet eight inches of water in 

 the ship channel. They consequently form an impediment to. 

 commerce. The depth is, however, always increased in the spring 

 season, when twelve inches more may be generally relied on. 

 Frequently, during the droughts of summer, a change of wind, 

 and its steady continuance for some time, will allow ships to pass 

 without lighters. The permanent removal of this bar is, however, 

 an object of national importance, which cannot but be felt, as the 

 tonnage of the lakes increases. 



Ancient Dune ; A Buried Forest. — The principal spot where 

 the lands, in the immediate vicinity of the water, assume any con- 

 siderable or abrupt elevation, is included between Black River of 

 the St. Clair and Lake Huron. Here the outlet of the lake, 

 which is rapid, washes the base of a ridge, or ancient dune, ele- 

 vated fifty or sixty feet above the water. Fort Gratiot occupies 

 the upper part of this elevation. The lower part consists of the 

 blue clay stratum, corresponding in character with that found in 

 the wells of Detroit. It is overlaid by a deposit of sand, forming 

 two-thirds of the entire height. This elevation is crowned with 

 a light forest of oak and other species. At the line of junction 

 between the sand and clay, a number of trees are seen to be hori- 



