CONTINENTAL TERRACES AND SUBMARINE VALLEYS IO9 



across the continental shelf almost to the fall-off where the 

 continental slope begins. 



The results of the investigation are summarized in the 

 lower cross-section in Figure 59. Here the top horizontal line 



FIGURE 59. EWING S SECTIONS OF THE CONTINENTAL SHELF, EASTERN 

 UNITED STATES. 



represents sealevel and the uppermost heavy line represents the 

 surface profile of coastal plain and terrace; Fort Monroe is 

 situated at the point marked FM. 



The refraction method demonstrated a discontinuity which 

 is indicated by the upper chain of small circles. Above the line 

 joining these points in depth, the wave-velocities were those 

 expected in loose sand, mud, and gravel. A second, much more 

 pronounced break of material was found at depths shown by 

 the lower chain of circles. Between those two discontinuities 

 the wave-velocities were those expected if the elastic waves had 

 been propagated in more compacted, perhaps semi-consoli- 

 dated, sediments of the same detrital kinds, though thin layers 

 or lenses of hard limestone may be interbedded. The diagram 

 shows a thickening of the terrace sediments from 3000 feet at 



