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and-south oscillations of the ice. When the ice finally retreated, the plants 
followed. As any area became warmer and drier, some species perished. 
The southern flora, long held in check by the glacier, began to crowd in and 
where conditions were favorable for its growth, replaced the arctic flora, 
which remained only in such situations as were unsuitable for the growth 
of the southern plants, such as bogs and cool, shady ravines. Such places 
as these are islands of northern plants left in our now southern and south- 
eastern flora. 
The physiographic cycle of a bog differs from that of an ordinary swamp 
in several particulars; while both are ephemeral features of the landscape, 
soon being destroyed by sedimentation or by drainage, they differ in the 
manner in which they are filled; a swamp fills up from the bottom by the 
gradual accumulation of sediment deposited by incoming streams and that 
formed by decaying plant and animal matter; while a bog fills largely from 
the top by the formation, beginning at the edge, of a gradually thickening 
and settling floating mat of partially decayed vegetation, which is finally 
capable of supporting a rich flora. Bogs are more likely to develop in un- 
drained or poorly drained depressions, though there are partially drained 
bogs and undrained swamps. 
The glacial age was not a unit, but was characterized by alternate ad- 
vances and recessions of the ice, repeated no one knows how often. The 
last few advances were, in general, less extensive than their predecessors, 
so the terminal moraine of each was not, in every case, destroyed by its 
successor. The moraines of three of these successive advances of the ice 
can be distinguished in Ohio (2). The oldest, the Illinoian, extended almost 
to the Ohio river. The second, the Early Wisconsin, extended nearly as far, 
and was divided by an elevation of land into two lobes, the Scioto on the east 
and the Maumee-Miami on the west. The Late Wisconsin sheet followed 
the same course as did its predecessor. The terminal moraines of the two sheets 
are roughly parallel. The medial moraine of the two lobes of the Early 
Wisconsin Sheet was not destroyed by the Late Wisconsin, and the 
outwash plain between the medial moraine of the Early Wisconsin and the 
lateral moraine of the Late Wisconsin formed a broad valley, now drained 
by the Mad river. In this valley is located a bog, known locally as the Cedar 
Swamp. See accompanying map, Fig. 1. 
Cedar Swamp is in Champaign county, Ohio, about five miles south of 
Urbana. It is between the river and the east bluff of the valley. There is 
