488 BOUNDARIES OF THE SEA 



that can be left to geologists to squabble over, but the geophysical 

 investigation of the internal structure is an essential requirement. 

 Marked tectonic activity of the coastal regions, both marine and 

 terrestrial appears to characterize this type of terrace. Curiously 

 enough such terraces are most usual along the borders of what 

 many geologists, for disputable reasons, claim to be the most 

 ancient ocean basin, namely the Pacific. 



The opposite type of terrace is typical of the Atlantic, especially 

 the eastern border of the Americas. There the probing geophysicist 

 tells us he encounters a broad shelf underlaid by a thick column of 

 sediment. The most probable structure appears to be a wedgelike 

 thickening offshore with the strata outcropping on the slope, per- 

 haps even bending up there. Axailable information is meager but 

 it indicates a Alcsozoic land surface sloping down to the floor of 

 the ocean, and covered by a mass of sediment thickening seaward 

 and then thinning again below the continental rise. Such a situa- 

 tion can hardly result as a primary accumulative form, even if we 

 assume gradual subsidence progressing outwards in amount. A 

 cutoff l)y a fault or by collapse, or erosion by turbidity currents 

 are concei^'able explanations between which a choice is as yet im- 

 possible. We do not even know whether the sediment collected on 

 an open shelf or whether some shelter was provided. 



A bafiling problem must be pointed out. The terrace along the 

 Atlantic border of the United States is among the largest in the 

 world. Yet it was built in about 4% of geological time. In the re- 

 mainder of the earth's history a number of such terraces must have 

 been constructed in succession around the continents. There is no 

 reason for postulating an exceptionally high rate of sedimentation 

 off New England compared with other continental margins. Either 

 those inferred and now missing ancient terraces have been in- 

 corporated in the high-standing continental blocks or they have 

 been engulfed in the oceans. Both suggestions are unsatisfactory. 

 Only a few continents have thick sedimentary prisms of sufficient 

 magnitude along their margins to be considered in this light as 

 former continental terraces. Moreover, in some cases the distribu- 

 tion of grain size denotes supply toward the continent; in others 

 the thickness decreases on approaching the ocean edge from land. 



