‘HURON, COUNTY. hee 291 
comminuted debris has oe intimately mixed with that of the granite 
of the north, and of all the intervening rocks, and the whole spread out 
over the surface of the county. As the waters which covered the surface 
at the close of the glacial epoch receded, each of the terraces described 
above was formed, and each for a long period constituted a shore swamp, 
‘in which the decomposing vegetable material accumulated to form a soil 
of unsurpassed and permanent fertility. : | 
The materials composing the upper terraces were long subjected to the 
action of shore waves, and in places the surface is occupied by sand dunes 
and assorted gravel. The lower terrace is a broad prairie, with swampy 
muck soil. When the country was first settled, some of this was not 
sufficiently reclaimed from the water to support forest trees, but the 
greater part of it is now remarkably fertile farming land, especially 
adapted to the cultivation of corn. In afew places the prairie soil rests 
.upon the bed-rock, but generally upon a heavy. deposit of boulder clay, 
containing the ordinary granitic and metamorphic boulders, and also a 
- great profusion of fragments of limestone; and wherever gravels are found 
a large percentage of the pebbles is derived from the limestone. Indeed, 
the Drift deposits here all contain an unusual abundance of the debris of 
_. the limestones. In the bed of Huron River are many large boulders of the 
Corniferous limestone. In the sand hills the cavities left by decaying 
roots are often filled with calcareous tufa, and crumbled and broken layers 
of the Berea grit in the quarries are frequently cemented into a coarse 
breccia by the percolation of linge water from above.. Also the beds of 
water-worn pebbles are here and here cemented into a conglomerate by 
the same cause. To these facts, together with the abundance of humus 
from the old swamps which once covered the surface, we must attribute 
the remarkable fertility of a large part of the county. 
The general elevation of the level. prairie land in Lyme township is 
one hundred and twenty-five feet above the lake.. “Here i isa succession of 
remarkable sand dunes which rise to the. height | of thirty feet. The sand 
_ composing them is fine, shows irregular, waved lines of stratification, and 
rests upon peat. These sand hills were formed, as was much ‘of the main 
sand ridge of the county, by wind and wave action. along the lake shore, 
and on the margin of a shore swamp caused by’ this barrier, in which 
vegetable debris accumulated fora long time. The swamp soil in many 
places contains abundant remains of corniferous trees, and extends under 
the dunes and the sand ridge, because the sand was drifted back from the 
' beach over it. The north and south bases of the ridge and sand hills 
have the same elevation above the lake. The north side of the ridge 
exhibits the irregular, winding outline of a lake beach, while on the 
