1905] TRANSEAUBOGS OF THE HURON RIVER VALLEY 361 



detachment of blocks and masses of ice through differential melting 

 (19). If these detached masses happened to be in the line of the 

 overloaded glacial drainage, they became covered to a greater or less 

 extent by sand and gravel. Owing to the poor conduction of heat by 

 such deposits, they melted with extreme slowness. Where this latter 

 process was prolonged until the drainage line had been abandoned 

 or the stream had ceased depositing, subsequent melting brought 

 about a settling of the deposits and the production of basins. Sister, 

 Kavanaugh, and Crooked Lakes are examples of this type. 



In the case of the chain of lakes which form a part of the Huron 

 River in northwestern Washtenaw County, and such lakes as Portage, 

 Tamarack, Ore, and Bass, according to LEVERETT, there was an 

 additional settling of the fluvio-glacial deposit itself. This latter 

 process has been of the greatest importance in the development of 

 extensive bog areas.' In the Portage Lake region this settling has 

 amounted to as much as 40 feet (i2 m ) in certain places, and has 

 resulted in reducing many hundreds of acres of land to the ground 

 water level. 



Throughout the belt of till plains occur shallow marshes, some- 

 times drained, but usually by a sluggish meandering stream, itself 

 impeded by the growth of swamp plants. These basins are the 

 natural expression of the unequal deposition of glacial material. 

 Till plains result from a comparatively rapid retreat of the ice ; hence 

 the depressions are usually shallow, and have been mostly filled with 

 peat to the level of the present drainage. The several small lakes 

 lying to the west of Dexter are examples of basins not yet obliterated. 



Where the retreat of the glacier is slow and deposits are made to 

 a great thickness about the edge of the ice, kame or "knob and kettle" 

 topography results. The basins of such areas are characterized 

 usually by high margins and comparatively steep slopes. West, 

 Silver, North, Island, and South Lakes may be cited as examples. 



As we know from remains discovered in peat deposits, among the 

 animals inhabiting this region in early postglacial times were the 

 mammoth, mastodon, bison, peccary (Platygonus compressus 

 LeConte) (57), elk, and u big beaver" (Castoroides ohioensis Foster). 

 The last named is not a beaver (34, p. 256), but is more nearly 

 related to the Coypu rat of South America. The common beaver 



