[30] In some cases these iron and manganese deposits are undoubtedly formed by the replacement of limestone or other rocks, as is further discussed on pages — to —.
[31] Kentucky Geol. Survey, Report of Progress, Vol. III., New Series, 1877, p. 164.
[32] U. S. Geol. Survey, Tenth Annual Report, 1888–1889, Vol. I., pp. 409–422.
SOME RIVERS OF CONNECTICUT.[33]
Outline.—Introduction.—Topography of Connecticut: The upland plateau, its origin, date, elevation, valleys sunk beneath its surface.—Lowland on the Triassic area.—Later oscillations.—Résumé of the topography.—Early drainage.—Re-adjusted streams.—Revived streams.—Unconformable rivers, consequent or superimposed.—Pleistocene changes; the Farmington, Quinnipiac, Scantic.—Abandoned gaps.
Introduction. In order to study intelligently the history of a river, one must first become acquainted with the present physical geography of the region in which the river lies, and know the stages of its development. Therefore, before classifying the rivers of Connecticut, I shall consider the topography of the state, and in a few paragraphs outline the successive cycles in the history of its growth. The scope of this article will not permit a discussion or even a full statement of the evidence on which these conclusions are based. They have been stated at considerable length by Professor W. M. Davis,[34] and the reader is referred to his papers for the complete discussion. His conclusions in respect to the physical geography are accepted here without question, and form the basis for the discussion on the rivers of the state.
FIG 2[35]
Topography of Connecticut. Connecticut can be said to consist of two great areas quite distinct in topography and geologic structure.[35] On the east and on the west are the crystalline uplands which rise from sea level along the Sound to 1,700 and 1,800 feet in the northwestern part of the state, and to 600 and 700 feet in the northeastern. These uplands consist chiefly of gneiss and granite, probably of pre-Paleozoic age, which are now much folded, faulted and crumpled. Between these two areas of crystallines is a lowland belt of Triassic sandstone and shale, twenty to twenty-five miles wide, extending from New Haven north through the center of the state and including in its borders New Haven, Meriden, Hartford, New Britain and many towns of lesser note. These sandstones form a monocline with an eastward dip of 10° to 30°, and in addition to being tilted they have been faulted since their deposition in a shallow, slowly-subsiding trough of crystallines. Their thickness is variously estimated—3,000 to 5,000 feet, Dana; 10,000 or more, Davis. This lowland is interrupted by a series of trap ridges, which in general present steep faces toward the west, whereas their eastward slope is gradual, less than the dip of the sandstones.