The Flora of the Cayuga Lake Basin 439 



tive of the rocks, in order that the reader may have a general understanding of the 

 conditions in the basin. 



The rocks underlying all of the Cayuga Lake Basin are unaltered stratified sedi- 

 ments belonging to the Devonian and Silurian systems. They consist of alternating 

 layers of sandstone and noncalcareous shales, the latter predominating, and hence 

 they are comprised of some layers of hard flagstone with much soft, friable shale 

 between. There are included also, especially in the Silurian system, certain layers of 

 limestone and a considerable amount of calcareous shale. To the horizontality of 

 position and the alternation in hard and soft layers, much of the peculiar character 

 of the ravines and their cataracts is due. The strata have a low and very uniform dip 

 toward the south, about three feet to the mile on the average though locally much 

 increased or even reversed in direction. This fact, combined with the general slope 

 of the surface topography toward the north, results in the exposure of lower geological 

 formations at the northern end of the basin. At the extreme northern end, on the 

 Ontario plain, the rocks are of the Upper Silurian system, in which limestones and 

 calcareous sandstones and shales predominate. The rocks of the plateau region 

 from Cayuga village southward are all of the Devonian age, and in their lower strata, 

 outcropping near the northern end of the lake, limestones, calcareous sandstones, and 

 calcareous shales also form a prominent feature. These layers at the north, out- 

 cropping across the country eastward and westward, are exposed southward only as a 

 narrow band on each side of the lake valley to within a few miles of the city of 

 Ithaca. The rocks farther south represent higher strata in the Devonian system, 

 and are chiefly noncalcareous sandstones and arenaceous shales. 



PHYSIOGRAPHIC HISTORY 



Plant distribution in the Cayuga Lake Basin, through its relation to soils and 

 topography, is indirectly related not only to the underlying rocks of central New York, 

 but also to the general physiographic history of the region, on which the topography 

 and soils depend. The material constituting the rocks of central New York was 

 deposited as marine sediments in the Paleozoic sea which stretched westward and 

 southward from the ancient Appalachian land mass of the east and north, now in 

 part subsided below sea level. At the end of the Paleozoic era the central New York 

 area was uplifted to a very considerable altitude. Tarr thinks that the subsequent 

 denudation, bringing the upper plateau to the present level, has been equal probably 

 to the present maximum height above sea level, or about 640 meters (2100 feet). 39 

 The uplift was probably intermittent, but the deformation was not great, as is shown 

 by the present nearly horizontal position of the strata. From the summits of the 

 higher hills it is immediately perceived that the tops of all the hills are at the same 

 general level and constitute a plain or a plateau into which the valleys have been 

 carved. The surface of the plain so reconstructed is therefore an old erosion surface. 

 The slope of the land toward the north has been assumed to indicate that a large 

 part of the drainage of the uplands was preglacially, as now, in that direction, 

 possibly along the axes of Seneca and Cayuga Lakes. The plain-like character of 

 large stretches of the upper plateau and again of the lower plateau is probably 

 due to wide areas of resistant underlying rock strata at different levels. The 

 denudation following the first uplift apparently continued until the whole region was 

 reduced to sea level, producing the erosion plain of the upper plateau mentioned 

 above and now represented by the level surfaces of the higher summits. Another 

 uplift, giving rise to the present elevations, followed. After that, stream erosion 

 once again carved broad mature valleys, now represented by the wide upper courses 

 of the larger streams, especially those with an east-west direction. 



^ Tarr, Ralph S. Physiographic record, p. 213. U. S. Geol. Survey. Geologic atlas of the 

 United States, Folio 169, Field ed. 



