SAGINAW LOBE. 149 



and southwest directly toward the main part of the moraine instead of parallel with it. Some 

 of the ridges are continuous for 3 to 4 miles and are loosely connected end to end for 10 to 12 

 miles. They rise 2Q to 40 feet and in places 60 feet above the bordering nearly plane tracts. 

 One of the most prominent, located 2 miles west of Bronson, was used as a site for a geodetic 

 station. It has an altitude of 998 feet above sea level and is bordered by a plain whose altitude 

 is 910 feet on the south and about 920 feet on the north. It forms the southwestern terminus 

 of a narrow ridge about 3 miles in length. 



In addition to the ridges and till plains this district, in western Branch County, is traversed 

 by gravel plains that also trend northeast and southwest or parallel with the ridges. One of 

 these gravel plains heads about 4 miles east of Colon and gradually expands westward to enter 

 the gravel plain of St. Joseph River at Colon. South of this and separated from it by a ridged 

 belt scarcely a mile in width is a longer and larger gravel plain that heads east of Matteson Lake 

 and includes the lake. A still larger gravel plain (2 to 4 miles wide) leading from Coldwater 

 River southwestward to Prairie River may have received an outwash from the moraine of the 

 Huron-Erie lobes on the east as well as from the receding Saginaw lobe. 



It is probable that all these drainage lines as well as the great gravel plain on the St. Joseph 

 extended headward with the retreat of the ice sheet. The smaller two seem to have ceased 

 to be functional before the ice had withdrawn, and the larger two to have remained in operation. 

 Whether the elongated ridges were formed by gradual extension from southwest to northeast 

 with the recession of the ice is not so clear. Indeed, their method of development is not as 

 yet understood. 



In the southern part of Branch County the ridges and knolls are distributed more irregu- 

 larly than they are in the western part and they inclose gravelly and sandy plains as well as till 

 plains. They commonly rise 20 to 30 feet above the plains and are»either isolated or in small 

 groups. 



West of Three Rivers, in the prominent interlobate part of the Sturgis moraine, much very 

 rough land is interspersed with basins occupied by lakes and marshes. Spurs of moraine run 

 out into the lake basins and sharp knolls appear in the midst of the basins. The high knobs 

 in central Newberg and southern Elowerfield townships are among the most prominent in 

 southern Michigan. A strip on the northwestern edge of the tract that includes these sharp 

 knobs is more choppy than the remainder of the moraine, possibly owing to overriding by the 

 Lake Michigan lobe after the melting of the Saginaw lobe. (See p. 152.) 



STRUCTURE OF THE DRIFT. 

 CHARACTER. 



The surface of the Sturgis moraine is almost everywhere thickly strewn with bowlders. 

 This is true not only of the main part of the moraine but also of the isolated or island-like tracts 

 and of the transition belts on the inner border. The bowlders number several thousand to the 

 square mile and are of various sizes up to 10 or 12 feet or more in cUameter. Their presence 

 in such numbers intensifies the morainic exj>ression even of a belt whose knob and basin topog- 

 raphy is strongly morainic. The bowlders, which include numerous quartzites and some 

 quartzitic conglomerates apparently from the Huronian formations north of Georgian Bay, 

 are chiefly of granite, as in moraines farther south and west. 



The sharp knolls and ridges of the moraine also carry a large amount of surficial cobbly 

 material, and the numerous shallow cuts and gravel pits indicate that considerable water action 

 attended their deposition. The beds show many interruptions and changes of dip and more 

 or less disturbance. The amount of clayey till is very small, at least in the upper part of the 

 drift, and there appear to be many changes both horizontally and in vertical section from loose- 

 textured till to assorted beds of gravel and sand and back again. Few wells go down more 

 than 60 feet and most of them only 20 to 40 feet, so the knowledge of the deeper part of the drift 

 is very meager. 



