Fig. 5. Cross section of Lake Cayuga Valley, two miles north of Ithaca. (Scale same as Fig. 4.
longer present. To state this explanation calls for a preliminary consideration of the general topography.
The Finger Lake valleys extend nearly north and south, long and narrow, like so many fingers, the two longest, Cayuga and Seneca, being about forty miles in length, and at the lake surface from one to three miles in width. Their bottoms are below sea-level. They are excavated in the plateau of southern New York, a dissected plateau of nearly horizontal Devonian shales and sandstones, trenched by many deep valleys (Fig. 2). But among these valleys those of the Finger Lakes stand out prominently because of certain notable peculiarities.
If one of the upland valleys of the plateau should be damned so as to contain a lake as deep as Cayuga (435 feet) or Seneca Lake (618 feet), or even one a hundred feet in depth, its shore-line would be very irregular, and its waters would extend as bays up the tributary valleys. But in the Finger Lakes this is not the case. The lake shores are smooth and regular (Fig. 3), and this condition extends for several hundred feet above the lake level. The valley walls enclosing the lakes are gullied only by the narrow gorges which are so abundant (Fig. 1). Not only are they smooth and regular, but they are steepened below the 900-foot contour line (Figs. 2 and 3), so that a profile of the valley slope shows a distinct increase in the steepness of the valley wall below that level (Figs. 4 and 5).
At the upper level of the steepened slope, upland valleys open out and lead back into the plateau, so that if the lake waters could be raised 500 feet higher than the present, they would enter into these tributary valleys and the lake shore line would become very irregular, as is natural in a stream valley dammed so as to hold a lake. These upland tributary valleys are broad and mature, having evidently required a long period for their formation; and their counterpart appears throughout the plateau region. They are the normal valleys of the region: the Finger Lake valleys the abnormal.