rarely sufficed to remove the drift sufficiently for much rockcutting, so that there are no post-glacial gorges, though there are many sags or small valleys representing post-glacial stream erosion. Near Groton on the east side of the valley two streams for short distances are on rock, but this is nearly a mile from the floor of the main valley. North of Centerville we find the gorge of Dry Run (fig. 6), already alluded to. On the west slope of the Owasco valley we would anticipate many glens, since the steep fall should encourage rapid erosion, but the creeks have such limited catchment basins that they have been unable to produce any marked channels in the slopes. At Montville the stream coming from the north flows in the last mile or so of its course, between rock walls. This stream has a fall of fifteen to twenty feet over the Tully limestone, which forms a conspicuous shoulder and is easily quarried along the east wall of the valley as far south as Locke; and a short distance down stream a slightly greater fall over hard layers in the underlying shale, the latter fall being used by the village of Moravia in connection with its electrical plant. Northwest of Lake Como is a slight post-glacial gorge cut in some of the harder layers of the Portage sandstone. The small basin of this stream is apparently all out of proportion to the gorge-cutting here present. The explanation of the condition, however, is apparent as one follows the highway toward North Summer Hill. Just east of this village, at about the head of the valley whose gorge we are describing, is found a loop of moraine (p. 366) marking a position where the ice stood for some time. The gorge-cutting was done when the valley was carrying a burden of ice-front drainage. About a mile east of Sempronius one passes between rock walls in following the highway into the Skaneateles valley. These rock walls cannot be connected genetically with present drainage; nor, from deductions that one would make, has the former development of drainage in the area developed the gorge. The only reasonable hypothesis for the gorge cutting here represented is that the erosion was done by ice-front waters, and this supposition is sustained by the nature of the channel which leads into the head of this rock gorge from the north (p. 432). In the southwestern part of the quadrangle near Asbury is another gorge which presumably represents the work of post Fig. 3. Southeast of McLean is a tributary valley of Fall Creek, which heads in Cortland county. This view looks along the axis of this tributary valley, showing its flattened cross-section as well as the maturity of the major valley. Fig. 4. Looking east across the Owasco valley at Locke; camera stands near the 1200-foot contour. Valley drift shows on both slopes. Wisconsin waters. This gorge continues westward into the Genoa quadrangle. In the discussion of post-Wisconsin carving it is apparent that no sharp distinction has been made between the work of immediately ice-front waters and the erosion-work of more recent streams. In all cases, except the downhill gorges associated with the glacially steepened valley walls, and the channels connected with the Skaneateles Inlet, both factors probably enter somewhat into the gorge-cutting. The Skaneateles Inlet channel, however, is purely the work of an ice-front stream. Fig. 5. Looking north through Skaneateles Inlet valley; camera stands near mouth of overflow channel. Small portion of lake shows in middle of picture; the heavily wooded slope paralleling eastern shore marks the upper limit of more vigorous ice-erosion. PRESENT POSITION IN DRAINAGE CYCLE. Aside from a few post-glacial streams now in rock there is very little degradational work being done at the present time in the area of this quadrangle. Streams of this type have a local baselevel due either to glacial overdeepening of the main drainage lines of which they are tributaries, or to the work of an abnormal quantity of water which their valleys carried in immediate postglacial times. Such channels, where topographic adjustment is in progress, exist at Peruville, a few between Locke and Moravia, at Montville, and in the short tributary valley south of Dresserville. The base-level of the present Owasco Inlet valley is Owasco lake, which is 464 feet above the level of Lake Ontario. The baselevel represented by Lake Ontario is far removed from becoming active in the drainage degradation of the quadrangle. Fall Creek, a tributary of the Cayuga valley over the eastern wall of which it now drops1 at Ithaca, controls a large portion of the Moravia quadrangle. But the base-level represented by the water in Cayuga valley initiates a new drainage cycle for such parts of the quadrangle as are drained by Fall Creek. A recent cycle is also in operation for the valley tributary to the Owasco Inlet at Moravia. In all other respects this quadrangle occupies a prematurely advanced stage in its drainage cycle. The former major drainage line, that is, the valley now controlled by the Owasco Inlet, in its southern part has been so aggraded by glacial deposits that many of the streams which preceding the ice invasion were doing erosional work have in the main ceased to be agents of disintegration. This glacial interference with the erosion cycle is the same in kind as has become operative in all of these Finger Lake valleys. It becomes apparent, therefore, that one of the results of glaciation is the hastening of the position which drainage in its normal development would have brought about. On the other hand, certain upland valleys contiguous to these major drainage lines have been started on an entirely new cycle through the erosive work of ice in the longitudinal valleys to which the upland valleys were pre-glacially graded. DISTRIBUTION OF THE Drift. GENERAL DISCUSSION. In accounting for the veneer or for the deeper accumulations of drift found in glaciated countries, one considers both the local topography and the topographical aspect of probably all the area 1 R. S. Tarr: Am. Geologist, vol. xxxiii (1904), pp. 271–91. |