PRICE 



29 



as if transversely entrenched stream valleys were cut during the last lowering of sea level. 

 The gullies have not been filled by sedimentation, nor have their divides been removed by 

 erosion. Some of these gullies are definitely shown by the soundings, and others inferred from 

 insufficient data. 



The floor at the upper end of the Mississippi submarine canyon is about 450 feet below 

 sea-level, which Fisk estimates to be the amount of the last great lowering of the sea. The 

 contours on Fig. 17 from U.S.C.&G.S. Chart 1003 show the deep delta of the Mississippi canyon 

 better than the older charts. The top seems to be about at 600 fathoms and the base at 2,000 

 fathoms. The fan extends out and down to meet the Yucatan shelf and the mouth of the straits 

 of Florida. Its front is some 300 miles wide. 



Figure 18 : The Yucatan area. The shoreface is 18 feet high, which is fairly low in com- 

 parison with those of 45 to 50 feet high common on most sandy coasts. The mangrove coast is 

 merely sketched in with only the entrances to the numerous tidal channels shown in a rough way. 



The submerged karst plain at the north slopes about two feet per mile. On the low-energy 

 coast at the west it slopes 1.3 to 1.9 feet per mile. The few elevations available on land on the 

 north coast indicate a slope of three feet per mile to the north, suggesting that the offshore 

 slope may be a partly built-up slope, rather than an erosion slope. 



Figure 18. 



