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The current interest in contemporary tectonic processes in the eastern United States is turning up abundant evidence of crustal movements in late geologic time. Topographic analysis of the highland areas from the southern Blue Ridge to the Adirondack Mountains indicates that most of the landforms owe their origin to erosion of rocks of different resistance rather than to tectonic processes. Most areas of high relief and high altitude have been formed on resistant rocks. The Cambrian-Ordovician belt, containing mostly shale and carbonate rock, on the other hand, forms an extensive lowland from Alabama to the Canadian border, and girdles the Adirondack Mountains. Differences in altitude can be explained by the presence of resistant rocks outside the belt; these resistant rocks form local base levels on the streams that drain the belt. A few areas may have undergone local uplift at a higher rate than areas nearby for example, the Piedmont region northwest of Chesapeake Bay. Most estimates of erosion rates based on the load transported by streams and of uplift rates based on removal of the known or inferred amount of overburden during a known period of time are of the same order of magnitude, averaging about h X 10~2 mm per year. Rates of uplift based on study of tilted Pleistocene beaches and repeated geodetic traverses are at least an order of magnitude higher for comparable areas. Tectonic uplift of the highlands has been slow and involves mostly warping or tilting on a large scale. Erosion rates keep up with or exceed the rate of uplift and have been sufficient to mask evidence of faulting or other differential movements. The high rates of uplift that are inferred on tilted water planes in the glaciated regions or that are measured by differences in repeated geodetic traverses cannot have been sustained for long periods of time. Differential erosion of rocks of varying resistance has long "been considered a major factor in shaping the landscape of the Eastern United States. Theories to explain the erosional history have "been concerned mainly with cyclical landform development, relict surfaces, such as peneplains, and the evolution of the drainage pattern (Thornbury, 1965, p. 72-87). Although most geologists agree that uplift did take place in the Eastern United States during late geologic time, that is, since the middle-Mesozoic, few have thought that local topographic forms are of tectonic origin. Faults active in late geologic time have been discovered (York and Oliver, 1976 j JYtixon and Newell ? 1977) "but it has not "been proved that any modern topographic features owe their origin to contemporary faulting. Recent studies of precise levelling data along geodetic traverses that have been rerun one or more times indicate that many of the monuments have changed in elevation during the interval between traverses. Some of these changes may be due to errors in levelling or to disturbances of the monuments, but it is generally agreed that many of the changes are related to actual motion of the ground or of the Earth's crust. The differences between traverses expressed as velocities of vertical motion are commonly quite large (see Brown and Oliver, 1976, for discussion of the method of analysis). The interpretation of the repeated traverses poses many problems. The rates of uplift or depression are commonly so high that they could not be long sustained without resulting in much larger topographic features than are observed. In spite of the difficulties involved in interpreting evidence for recent crustal movements, there is little doubt that rates obtained from precise levelling data as well as from study of ancient strand lines indicate rapid contemporary crustal movements. Analysis of topography of the Appalachian Highlands shows, however, that differential erosion of rocks of different resistance is the major factor that controlled the development of today's topographic forms, including even large features. A few major topographic discontinuities apparently unrelated to rock type do exist, however, and can be identified. The picture that emerges from topographic analysis is that the major drainage system of the area has through long periods of time become closely adjusted to rock type. In places, it still deviates from lithologic controls. Some of the deviations occur in response to competing major drainage systems that have different base levels, lengths, and gradients, which are basically caused by rock control within the different systems. Some may be due to tectonic influence, and others may be inherited from the past. The rate of topographic adjustment because of erosion is so rapid that detection of the details of tectonic control is difficult or impossible even in areas where it is reasonable to expect that such control exists. On the other hand, the sedimentary record preserved in the Costal Plain and Continental Shelf as well as the grossest topographic relationships are clear evidence of tectonism. The intent of this paper is to describe the major drainage systems and topographic forms of the Appalachian Highlands between Alabama and northern New York and to discuss their adjustment to bedrock. Some topographic anomalies that may be caused by differential movement of separate tectonic blocks are also considered. I wish to thank C.S. Denny, Richard Goldsmith, Sheldon Judson, and Wayne Newell for their thoughtful criticism. Major topographic features and drainage systems The general outlines of the topography and drainage of the Appalachian Highlands are shown in figures 1 and 2. In figure 1, an envelope map originally defined by Stearns (1967), contours are drawn across the ridge crests and drainage divides. Essentially, figure 1 is a map of the high parts of the topography. In this paper, the envelope map is used as a reference map showing the major streams and the continental divide separating the drainage to the Atlantic Ocean from that to the Gulf of Mexico and St. Lawrence River. Figure 2, a subenvelope map (Stearns, 19&7; Hack, 1973) is another way to generalize the topography. In this map, the contours are drawn on altitudes along the major streams; it shows the position of the major drainage divides as well as the general configuration of the topography. The degree of generalization is controlled by the spacing of streams used in the generalization. The streams chosen can be selected precisely simply by eliminating the upper reaches of a certain length of all the streams. The length of reach chosen for elimination determines the degree of generalization. The subenvelope is useful for the present purpose because the larger streams are flowing on bedrock, and their channel gradients are adjusted to the bedrock.

Rock control and tectonism: their importance in shaping the Appalachian Highlands