Rocky River is classed with streams which are comformable to the rock
structure. This conclusion rests largely on the analogy between Rocky
River and other rivers of this region. The latter very commonly are
located on belts of limestone, or limestone and schist, and their
extension is along the strike. The interfluvial ridges are generally
composed of the harder rocks. The valleys of the East Aspetuck and
Womenshenuck Brook on the north side of the Housatonic, and of the
Still, the Umpog, Beaver Brook, the upper Saugatuck, and part of Rocky
River are on limestone beds (fig. 2). In the valleys between Town Hill
and Spruce Mountain (south of Danbury), two ravines northwest of
Grassy Plain (near Bethel), and the Saugatuck valley north of Umpawaug
Pond, the limestone bed is largely buried under drift, talus, and
organic deposits, but remnants which reveal the character of the
valley floors have been found. The parallelism between the courses of
these streams and that of Rocky River and the general resemblance in
the form of their valleys, flat-floored with steep-sided walls, as
well as the scattered outcrops of limestone in the valley, have led to
the inference that Rocky River, like the others, is a subsequent
stream developed on beds of weaker rock along lines of foliation.
[Illustration: ~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.
[Footnote 5: Gregory, H. E., Robinson, H. H., Preliminary geological
map of Connecticut; Geol. and Nat. Hist. Survey. Bull. 7, 1907.]