4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I49 



made by the U. S. Geological Survey, the Mississippi River Commis- 

 sion, and the Corps of Engineers of the U. S. Army. On many of 

 these maps the topography is shown by 5-foot contour lines. On such 

 maps boundaries between terraces can be traced with considerable 

 precision. Because of their great lateral extent and the variation of 

 slope of the land at them, shore lines are much more readily traced 

 on topographic maps than by examination in the field, where the range 

 of view is limited. 



FLUCTUATIONS OF SEA LEVEL 



A dominant cause of emergence and submergence of the land is 

 changes in sea level. Such changes raise or lower the shore line on 

 all islands and continents to the same extent wherever the land has 

 remained stationary. With each change in level the shore line migrates 

 seaward or landward, the distance depending on the slope of the 

 adjacent sea bottom or on the topography of the land. 



The Quaternary Epoch was a time of unstable sea level. There 

 appears to have been an intermittent lowering throughout the epoch, 

 reflecting increases in the capacity of the oceanic basins caused by 

 down warp of the bottom of the sea. There also were variations in 

 the volume of water in the seas resulting from changes in the size 

 of the continental ice caps. During glaciations there was less water in 

 the seas, and sea level was temporarily lower. The resultant of these 

 two causes — glacial control and increase of capacity — was repeated 

 fluctuations of sea level. 



As the number of shore lines recognized exceeds the number of 

 generally accepted interglaciations, which were times of high water, 

 some of the shore lines presumably represent intermediate still stands 

 during the intermittent lowering of sea level from causes other than 

 glacial control. The terrace deposits that accumulated during such 

 intermediate stands should lie conformably on the deposits, if any, 

 of the next older terrace, though there might be local unconformities 

 at the lower shore line caused by coastal erosion. The major discon- 

 tinuities within a sequence of terrace deposits should mark the 

 advance of the sea across the land after each glaciation. 



(It should be noted that terrace deposits may be absent from 

 places distant from sources of sediment. A discontinuity at the base 

 of a terrace deposit may indicate merely the lack of sedimentation 

 during the preceding epoch, not erosion.) 



Many attempts, none very convincing, have been made to correlate 

 the emerged shore lines with the chronology of the glaciated regions 



