The Western Highland Rim in central Tennessee is the upland physiographic province between the Central Basin on the east and the Mississippi Embayment on the west. In this area, there is close similarity in the configuration of the upland, the structure of the base of the Cretaceous, and the erosional levels of graded streams. Two alternative causal connections are likely: either (1) more recent steep-sided valleys are incised in old wide gentle valleys cut before Middle Cretaceous time by the same streams; or (2) the stream-flow system extends its erosional influence beneath the rim, and the inward-sloping surface of the old upland (capped by Cretaceous outliers) sags toward the incised streams because of ground-water sapping. The author prefers the second explanation. An unlikely third possibility is that streams flow in synclines folded after development of the upland.
In order to compare upland levels, bottom land levels, and Cretaceous structure (each having a different density of basic information), the writer used the envelope and subenvelope, which are arbitrarily generalized reproducible contour maps. The envelope (a device already in widespread informal use by geomorphologists) is a surface over the topography that graphically “fills in” valleys and sags. It limits topography on the high side and is regarded herein as an attempt to restore the topography to an hypothetical earlier stage before the valleys were incised. The subenvelope (newly devised for this paper) is a surface passing beneath the land surface, graphically “tearing away” hills. It limits topography on the low side and may be regarded in this case as an attempt to predict topography that may develop if the stream system continues a long time at its present grade.
Parallelism of contours developed by these techniques is consistent with either ground-water sapping or a rejuvenated stream cycle. The length of time (about 70 m.y.) elapsed since the development of the upland, however, seems excessive for the supposedly new stream peneplain to be so little developed, and a stream-developed-peneplain origin for the relatively level Highland Rim surface is doubtful.