Fig. 2. Geological map of Still River Valley.
The Geological Map of Connecticut[5] shows that the valleys of Still River, Womenshenuck Brook, Aspetuck River, and upper Rocky River are developed on Stockbridge limestone. The lower valley of Rocky River is, however, mapped as Becket gneiss and Thomaston granite gneiss. Although the only outcrops along lower Rocky River are of granite, it is believed that a belt of limestone or schist, now entirely removed, initially determined the course of the river. The assumption of an irregular belt of limestone in this position would account for the series of gorges and flood plains in the vicinity of Jerusalem bridge and for the broad drift-filled valley at the mouth of Rocky River. These features are difficult to explain on any other basis.
[5] Gregory, H. E., Robinson, H. H., Preliminary geological map of Connecticut; Geol. and Nat. Hist. Survey. Bull. 7, 1907.
JUNCTION OF ROCKY AND HOUSATONIC RIVERS
One of the distinguishing features of Rocky River is the angle at which it joins the Housatonic ([fig. 1]). The tributaries of a normal drainage system enter their master stream at acute angles, an arrangement which involves the least expenditure of energy. Rocky River, however, enters the Housatonic against the course of the latter, that is, the tributary points upstream. Still River and other southern tributaries of the Housatonic exhibit the same feature, thus producing a barbed drainage, which indicates that some factor interfered with the normal development of tributary streams. Barbed drainage generally results from the reversal of direction of the master stream[6], but it is impossible to suppose that the Housatonic was ever reversed. As will appear, it is an antecedent master stream crossing the crystalline rocks of western Connecticut regardless of structure, and its course obliquely across the strike accounts for the peculiar orientation of its southern tributaries, which are subsequent streams whose position is determined by the nature of the rock. For the same reason, the northern tributaries of the Housatonic present the usual relations.
[6] Leverett, Frank, Glacial formations and drainage features of the Erie and Ohio basins: U. S. Geol. Survey Mon. 41, pp. 88-91, figs. 1 and 2, 1902. See, also, the Genoa, Watkins, Penn Yan, and Naples (New York) topographic atlas sheets.