MORAINIC SYSTEMS AT HEADS OF LAKE MICHIGAN AND SAGINAW BASINS. 203 



CHARACTER AND DISTRIBUTION. 



A few elongated gravel ridges of esker type terminate in this interlobate system and 

 numerous short ridges of similar form and constitution which should probably be classed with 

 eskers. The short ones may be passed with the mere statement that they occur at various 

 points in the Saginaw part of the interlobate tract from eastern Livingston County northeast- 

 ward across northeastern Oakland County. Most of them are only 10 to 20 feet in height, 100 

 yards or less in width, and a mile or less in length. They commonly lie in swampy tracts, though 

 some of them are on the slopes of the prominent parts of the moraine. They are composed of 

 well-assorted gravel and are often drawn on for road material. 



LIMA ESKER. 



The Lima esker is the only prominent representative of this class of glacial ridges noted 

 on the Huron-Erie side of the interlobate tract. It is situated 10 to 15 miles west of Ami Arbor 

 in Lima Township, Washtenaw County, along a branch of Mill Creek that forms the outlet of 

 Fourmile Lake. (See Ann Arbor folio.) Its length is about 6 miles, its eastern end being in 

 sec. 24 and its northwestern end in sec. 4, Lima Township. It runs west for about 3 miles 

 and then near Lima Center makes an abrupt turn to the north; aside from this deflection it 

 follows a somewhat winding course, and in one place about \\ miles north of Lima it describes a 

 letter S. It rests on the uneven surface of the slopes of the valley or depression through which 

 the outlet of Fourmile Lake flows, rising and falling 30 feet or more with the surface on which it 

 lies. Its relief in few places is more than 20 feet, and it is interrupted by several small gaps. 



Gravel pits opened in the esker indicate that it was formed by a stream flowing west and 

 north, or in a direction opposite to that of the present drainage. At its northwest end is a 

 sharp gravel knoU about 50 feet high which it is thought may have been built by the same 

 drainage that formed the esker. It seems probable that the esker was deposited at a sufficient 

 height above the base of the ice to correspond with the height of this knoll, though it is possible 

 that water might gush up at the edge of the ice, as was noted by Russell in the Malaspina Glacier, 

 and deposit material there at a level higher than that of the tunnel through which it was flowing. 



The material of the esker is not markedly waterworn, and the preservation of fragile pieces 

 of Devonian shale of considerable size in it is thought to indicate that some of the material may 

 have been subjected to but slight transportation after it was released from the ice. The esker 

 development seems to have been the closing event of the subglacial drainage, its material being 

 deposited along the line of the drainage because the stream had insufficient strength to carry 

 it to the ice border. 



ACKERSON ESKER. 



The gravel ridge to which the name Ackerson esker is applied was noted in the dis- 

 cussion of the interlobate spur in southern Jackson County. (See p. 197.) It seems to stand 

 very near the junction of the Saginaw and Huron-Erie lobes and terminates in the morainic 

 spur formed between them. It passes from the west end of Wolf Lake past Ackerson Lake 

 through a gently undulating till plain and strikes the interlobate spur about 8 miles south of 

 Jackson. Its structure from top to bottom is revealed in a cut on the Cincinnati Northern 

 Railroad near Ackerson Lake. It contains very coarse material — much coarser than is ordina- 

 rily found in eskers — reaching a maximum in small bowlders nearly a foot through and slabs 

 2 feet or more long. The coarse material is quite generally present, especially southwest of 

 Ackerson Lake. The ridge is also more massive than the ordinary esker, being in places about 

 one-eighth mile in width, but it has the abrupt embanknieut-like appearance and the customary 

 great length of ridges of this class, being nearly continuous for about 7 miles and standing 30 

 to 40 feet above the border tracts along much of its course. Possibly it is the product of a 

 combination of ice deposition with subglacial drainage, and this may account for the coarseness 

 of its material and for its unusual size. 



