1995) TRANSEAU—BOGS 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 4o feet (12™) 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- 
limes 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 _ 
Ying 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 
“great thickness about the edge of the ice, kame or “knob and kettle” 
‘opography results. The basins of such areas are characterized 
; » Mastodon, bison, peccary (Platygonus compressus 
te) (57), elk, and “big beaver” (Castoroides ohioensis Foster). 
- last named is not a beaver (34, p- 256), but is more nearly 
ted to the Coypu rat of South America. The common beaver 
