MICHIGAN ACADEMY OF SCIENCE. 121 



IV. A REJUVENATED RAVINE. 



Of greater interest from a dynamic point of view, is a ravine situated across 

 the river, on the side opposite the city water works and near Foster. The 

 surface of the region in that vicinity is typically morainal. The ground is 

 undulating and hilly, the highest point running up to 300 ieet (90 m.) above 

 river level. The hills are rounded and at times occurring in parallels. The 

 depressions are of like characteristic. ]Many of them are occupied by swamps 

 or show de])osits of peat several feet in thickness and now covered by soil of 

 similar depth. Springs occur, but running water and streams are few, 

 scarcely any outlets being visible except such as have been produced through 

 ravine erosion. 



The drainage from these hills finding its way into the main ravine channel 

 is comparatively large. The soil is heterogeneous, consisting principally of 

 sand and gravel and boulder clay underlying the whole region, with frec[uent 

 pockets of fine sand or extensive beds of marl. Ground w\ater level and 

 water-content of soil vary accordingly. The seepage springs along the soil 

 line plainly reveal the importance of soil water-content as an influence in 

 the distribution of plants. The ravine has a length of about half a mile 

 (3 km.), but only a part of this is wooded. At the mouth the width is nearly 

 450 feet (136 m.), the ravine fronting the river from a direction north to 

 south. It seems that a phase of ravine life existed here similar to the one ■ 

 last described; and that at some not distant period a sudden increase of 

 drainage area occurred, which, no doubt, was due to the tapping of the 

 depressions and swamp areas north of the bluff. The effect of cultivation 

 has been scarcely less significant. 



That the earlier condition was but temporary is best seen in the changes 

 now taking place. Increased drainage and the consequent entrenched 

 meandering of the ravine stream — averaging in some places a depth of 5 to 

 6 feet — again are causing vertical banks, both in the old flood-plain of the 

 ravine as well as in the adjoining slopes. In these, though of a tenacious 

 blue clay, the eroded parts average a height of 15 to 25 feet. The size 

 of the trees, moreover, is clearly indicative of former favorable conditions, 

 while the activity of the erosion forces now present, is revealed by leaning 

 and fallen trees. 



At the mouth of the ravine occurs an extensive zone of sedges and ^villows. 

 An interesting feature in the sedge zone is the relative abundance of Chara, 

 the. relation of which to local marl formation has been recently described 

 by Davis (10). The plant gets its lime from the water and forms large patches 

 of fine-grained marl ileposits. Where the water comes with considerable 

 calcareous matter from the neighboring marl-loeds, incrustations develop 

 aroimd roots and mosses. Nearer the ravine stream Salix petiolaris. S. ros- 

 trata, S. lucida, Populiis tremuloides, together with Cornus stolonifera, 

 various species of Crataegus, Vitis vulpina and Celastrus scandens form a 

 loose thicket. The ground is little shaded and such plants as Gentiana 

 crinita, G. quinquefolia, Aster punicens. Prunella vulgaris, Euonymus obo- 

 vatus, Parnassia Caroliniana, Impatiens fulva, Urticastrum divaricatum, 

 Nabalus albus and Cystopteris fragilis are frequent. Here especialh' the 

 relation of the impervious clay layer beneath to the distribution of these 

 plants is evident. On the banks about the mouth Vaccinium vacillans and 

 Gaylussacia resinosa are dominant. 



10 Davis, C. A. "A Contribution to the natural history of Marl," Geol. Surv. of Mich' 

 8:65-90-1900-1903. 



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