Figure 4. — Percentage of tbe variance of the isobath depth 

 error that can be attributed to observational errors 

 (OBS), positional errors (POS), and interpolation 

 errors (INT). 



The figure is based on the variance in 100 repre- 

 sentative 5-miniite unit areas. In most of these unit 

 areas more than half of the total variance results 

 from uncertainty in interpolations of assumed 

 depths between the original soundings. 



Interpolation errors are \ery sensitive to topo- 

 graphic slope and spacing between soundings. To 

 achieve the same reliability of isobath depths, the 

 spacing of soundings must be much closer in areas 

 of steep slopes than in areas of gentle slopes. For 

 many surveys, not only of bathymetry but of other 

 variables as well, the observations are so widely 

 spaced (usually for reasons of economy) that ordi- 

 nary positional and observational errors have little 

 effect on the reliability of the final isolines. 



Figure 5 shows the relation of the standard 

 deviation of the isobath depth error to the meau 

 topographic slope in a few selected unit areas. The 

 dispersion of the points results largely from varia- 

 tions in the spacing between tracklines. 



The standard deviations of the isobath position 

 error (fig. 3) also increase offshore, mostly because 

 of wider spacing between tracklines. Figure 6 

 shows the relation of these standard deviations to 

 the mean spacing between tracklines. The disper- 

 sion of the points is due mainly to variations in the 

 observational and positional errors of the original 

 soundings. 



PART 2. GEOMORPHOLOGY 



A description of the configuration of the Middle 

 Atlantic Continental Shelf, along with a general 

 discussion of its evolution, of the processes invoh-ed 

 in its formation, and of its sediments, is presented 

 in this part of the report. 



GENERAL CONSIDERATIONS 



The general appearance, geological history, sedi- 

 ment distribution, and geomorphic processes of the 

 Middle Atlantic Shelf are discussed in this section. 



General Appearance and Past Geologic History 



The present continental border of eastern North 

 America can be divided into five geomorphic zones 

 which roughly parallel the present shoreline (fig. 

 7 ) : ( 1 ) a hilly to mountainous system of parallel 

 valleys and ridges (the Newer, or Folded, Appa- 

 lachian Mountains), (2) a fiat to hilly upland re- 

 gion (the Older Appalachian Mountains), (3) 

 a coastal lowland, in places submerged below pres- 

 ent sea-level (the Atlantic Coastal Plain and Con- 

 tinental Shelf), (4) a .submerged slope about 1,500 

 fm. high (the Continental Slope), and (5) a very 

 gently sloping surface merging seaward with the 

 deep ocean floor (the Continental Kise) . For a dis- 

 cussion of these geomorphic divisions see Fenne- 

 man (1938), Heezan et al. (1959), and Hammond 

 (1964). 



Before the Cretaceous Period (some 136 million 

 years ago) a succession of evolving highlands oc- 

 cupied the present sites of the Older Appalachians 

 and the Coastal Plain. According to Dietz and 

 Holden ( 1966) , these highlands were formed when 

 material uplifted from an ancient Continental 

 Slope and Rise and from the adjacent deep-sea 

 floor was added to a then smaller continent. This 

 process of accretion is supposed to have started in 

 the late Ordovician Period (about 445 million 

 years ago) and to have continued until the end of 

 the Permian Period (about 225 million years ago) , 

 eventually adding some 150 to 400 or more miles 

 to the continent. (Dates are from Kulp, 1961, and 

 Harland, Smith, and Wilcock, 1964.) The eroded 

 remnants of tliese old highlands now underlie 

 Cretaceous and younger sediments on the present 

 Coastal Plain; they outcrop in a belt of greatly 

 deformed and altered rocks throughout tlie Older 

 Appalachian Mountains. 



The region west of the Older Appalachians was 

 occupied in pre-Ordovician times by an ancient 



48 



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