STRATIGRAPHIC CLASSIFICATION STRUCTURAL CRITERIA 451 



A shows a formation transgressing from a broad sliallow basin across the 

 eroded edges of beds in a deeper syncline, as, for instance, from tlie Allegheny 

 basin eastward over an Appalachian Valley trough. The unconformity at the 

 base of this overlapping formation varies greatly from place to place in the 

 angle of divergence of beds on its opposite sides and in the time value of the 

 hiatus. At c the structural discordance is much greater than at a, h, and (/, 

 though at each of the latter points the hiatus is greater than at c. 



B shows an "angular" unconformity with a limestone resting on the eroded 

 edges of a tilted sandstone formation. The contact is irregular because of ine- 

 quality of resistance offered to erosion l)y the underlying formation, and the 

 depressions may or may not contain accumulations of clastic material. Often 

 they are swept entirely clear of such matter by the advancing sea. An uncon- 

 formity like this is seen at Rondout, New York, where late Silurian limestone 

 overlaps tilted late Ordovician sandstone. 



G shows an unconformity between two limestones, as between the Kinnnswick 

 and Fernvale limestones south of Saint Louis, Missouri. In these cases, as 

 shown in the right half of figure, the unconformity is often* barely distin- 

 guishable from an ordinary bedding plane. At other places, however, the sur- 

 face of the lower formation is more uneven and occasionally exhibits solution 

 cavities or channels, which are filled with sweepings of the old surface, or even 

 by ordinary marine deposits not seen in adjacent parts of the section. 



D illustrates stratigraphic conditions observed on the southern slope of 

 Ozarkia, and shows how the major unconformity on the right, where the Boone 

 rests on the Jefferson City, wedges apart so as to permit the intercalation of 

 numerous other overlapping formations that are intermediate in age and simi- 

 larly unconformable in their relations to each other. Obviously the time value 

 of the hiatus separating the intercalated wedges is greatl.y inferior to that of 

 the combined hiatus found toward the summit of the island ; but in physical 

 manifestation and structural discordance one may be as clearly defined Jis 

 another. 



E shows a cross-section of the Foster Mountain embaynient, north of Bir- 

 mingham, Alabama. This was excavated in the western slope of the Rome 

 Barrier and subseipiently occupied by a small arm of the Ordovician sea com- 

 ing in from the west. This occurrence is important in proving that consider- 

 able relief and consequent surface erosion took place occasionally in the Ap- 

 palachian Valley as far back as pre-Ordovician times, and in showing (1) that 

 the heavy chert conglomerate was dropped close to the shore, and (2) that the 

 argillaceous near-shore deposit may give wa.y to pure limestone deposition — in 

 this case it does as a rule — in less than half a mile from the shore. Occa- 

 sionally small chert pebbles were carried much farther out. 



