Ecology of the Cincinnati Region 
209 
sions, that on rock bluffs is the slowest and includes the greatest number 
of stages. Soil or rock is here influencing the succession, because of 
its resistance to erosion, and its effect on soil water. 
Hydrarch successions are of two kinds, those of undrained and those 
of drained or half-drained situations. The former are exhibited in three 
widely different topographic situations, namely, on uplands, in fijled 
valleys, and on flood plains; the latter, along the margins of streams, 
where the typical flood plain succession is in progress, and occasionally, 
in the pre-glacial filled valleys. During the hydrophytic stages, these 
two kinds of successions are characterized by different types of vege¬ 
tation. 
The hydrarch succession of the upland depends upon topography 
and soil. An undissected flat, combined with an impervious soil pro¬ 
duces the moist conditions necessary to the growth of hydrophytes. 
That soil is an important factor in producing these conditions, may be 
learned by comparing flats having different soils. Hydrophytic associa¬ 
tions are best developed on uplands covered with a relatively thick 
deposit of Illinoian drift. Where this deposit is very thin or absent, and 
only the loess (white clay) reaches its usual thickness, as in the southern 
part of the region, decidedly hydrophytic forests are not found. Only a 
few hydrophytic trees occur in an otherwise mesophytic forest. South 
of the limits of glaciation and the extent of loess, wet upland flats appear 
to be uncommon. Owen ( 1857 ) mentioned the “soggy beech flats” in 
Fayette County, Kentucky. There, as in this region, they seem to be inti¬ 
mately related to soil conditions, i. e., the sub-stratum is composed of 
fine-textured mudstones and shales, which produce a poorly drained soil. 
On comparing an area covered with till of Illinoian age with one cov¬ 
ered with the Earlier Wisconsin, a similar, but less pronounced difference 
is seen. Thus the imperviousness of surface material on the uplands 
modifies the character of the vegetation through its effect on soil water. 
The hydrophytic associations of undrained situations in the filled 
valleys are similar in character and causal relations to those of the 
uplands. 
The depression hydrophytic forests of the flood plain occur in habi¬ 
tats formed by streams, but now removed from the influence of their 
activity. Soil is not the important factor here. Low situation in combi¬ 
nation with undrained conditions determines their existence. 
