STRATTGRAPHIC CLASSIFICATION STRUCTURAL CRITERIA 459 



finer particles were probably carried seaward and later deposited with the 

 normal sediments of the time. If proper clastic material was available, 

 then it was arranged to form a beach. If not, then whatever littoral 

 fauna was present attached itself to or bored into the bare rocky floor, 

 and was finally buried beneath normal marine sediment in which land 

 detritus constituted but an insignificant part. 



Time values of stratigraphic hiatuses. — The chief difficulty about 

 overlaps and unconformities lies in the determination of the time value 

 of the hiatus. None of the physical criteria are competent to give a 

 reliable clue to its durance at any given point. A break corresponding 

 to several geological periods may be no more clearly marked than the 

 relatively brief interruption of sedimentation between two small forma- 

 tions or between diastrophically distinguished members of a single forma- 

 tion. In fact, in equally good exposures it is more difficult to detect the 

 contact between lower Pennsylvanian and Ozarkian sandstones in the 

 vicinity of Bolivar, Missouri, than to find the boundary between the Jef- 

 ferson City and the Yellville, in Arkansas, or the sharp line between 

 early and later limestones of the Black Eiver group at many places in 

 the Mohawk Valley in New York. It makes little difference whether 

 the compared sequences are lithologically similar or dissimilar; the large 

 hiatuses are, as a rule, no more clearly defined than the small breaks. 



Faunal evidence alone gives an immediate clue to the time value of the 

 hiatus. Wanting this, a fair idea of its relative importance may be 

 gained by tracing the break into areas where the sequence is more com- 

 pletely developed. The stratigraphic method is illustrated by figure D 

 on page 450, which shows how various formations are locally intercalated 

 in the hiatus between the Boone and the Ozarkian Jefferson City dolo- 

 mite in north Arkansas. 



Illustrative examples. — Six additional examples may be cited, with 

 barely sufficient discussion of their salient features to illustrate the more 

 commonly observed differences in unconformable stratigraphic bound- 

 aries: 



The first is the Ordovician Foster Mountain embayment, which is shown 

 in cross-section in figure E, on page 450, and is sufficiently described 

 in the legend accompanying the plate. It illustrates deposits in a sub- 

 merged valley, the bordering lands of which were comparatively steep 

 and supplied an unusually large amount of loose surface material. The 

 example is important, because it shows that in a rapidly deepening sea, 

 even under such favorable conditions as regards supply, the fine as well 

 as the coarse matter of the regolith is likely to be dropped within short 



