PIEDMONT TERRACES OF THE NORTHERN APPALACHIANS 689 



stratigraphic and structural evidence to elucidate the post-Jurassic history of 

 the region. 



Students of the strata of the Coastal Plain have discovered numerous sur- 

 faces of unconformity in those deposits, indicating repeated retreats of the 

 sea ; but they haVe drawn their shorelines close to the present outcrops, thus 

 expressing the conclusion that the oscillations of the strand have been essen- 

 tially confined to the present coastal plain, including the portions now sub- 

 merged. The geologist who has carried the evidence of the sea farthest inland 

 is W. M. Davis, who showed from the character of the superimposed drainage 

 that the sea had at one time advanced over the trap ridges of New Jersey onto 

 the border of the crystalline highlands. In the present paper it will be argued, 

 however, from additional lines of evidence that earlier invasions of the sea 

 advanced much farther inland, the farthest reaching in fact to central Penn- 

 sylvania and western Massachusetts. 



As bearing on the nature of the evidence for determining the limits of 

 former marine invasions the address of Gilbert is recalled, delivered to the 

 Geological Society December 30, 1892. In this it is noted that the present 

 limits of sedimentary formations are primarily determined by the length of 

 time which their outlying portions have been above baselevel, secondarily on 

 the thickness, resistance, and original limits. It is seen, therefore, that the 

 mere absence of Coastal Plain deposits from the higher slopes of the Piedmont 

 Plateau or seaward slopes of the Appalachians raises no presumption either 

 for or against the former presence of the sea over that region. If they had 

 once been present they would now be absent. The problem must, therefore, be 

 approached through other lines of evidence. 



Difficulties are then noted in the explanation of the higher surfaces planed 

 across resistant formations over certain parts of Pennsylvania, New Jersey, 

 and Connecticut as remnants of a single peneplain of subaerial denudation, 

 commonly spoken of as the Cretaceous peneplain. 



As a preliminary step for the reexamination of the problem a statement of 

 criteria is then given for determining with some accuracy where the evidence 

 is preserved, the elevation of former baselevels of subaerial and submarine 

 erosion. In addition to other criteria, it is shown that wind-gaps of a certain 

 character cut through hard ridges give the best record of subaerial baselevels. 

 The submarine plains are best discriminated through a determination of their 

 shores. For such plains as are shown by various lines of evidence to be of 

 marine origin the ancient levels are best determined from the marked accord- 

 ance and flat tops of the remnants of the more resistant formations. 



To detect by means of these criteria the existence of marine plains which 

 have been uplifted and very largely destroyed by subaerial erosion a method of 

 projected profiles is employed. This is applied to western Connecticut and 

 Massachusetts, and brings to light (he fact thai the plateau sloping from the 

 Green Mountains and Taconic Range t<> Long Island Sound is not a simple 

 Uplifted and dissected peneplain, hnt consisted originally <>\' a fllghl Of rock 



terraces, themselves somewbal sloping. The lower of those are relatively oar 

 row and poorly developed, hip preserve o considerable approximation to the 

 original surface. The higher terraces, originally wide and flat, were su< 



sively raised :ind \\:irped so long ago that t he\ have been \ery largely de- 

 stroyed. Certain Intermediate terraces are the most unmistakable in their 



