20 The Ocean 



are closer to reality than that in the upper diagram. A quantitative reading of differ- 

 ences in height is, however, hardly possible here, and with a vertical magnification of 

 only X 3 the thickness of the thinnest lines on the diagram is significant. The appear- 

 ance of prominent features such as the Whalefish ridge can scarcely be seen and any 

 qualitative differentiation into areas of greater or lesser irregularity is hardly possible. 

 Magnification of the vertical scale is thus necessary from the topographical point of 

 view, but must be used with appropriate caution. 



No accurate numerical evaluation of the echo-sounding profile, in order to fix the 

 degree of bottom irregularity in different parts of the ocean, has yet been made. The 

 superficial appearance of most of these profiles shows that the bottom relief varies 

 from one area to another, and care is needed in making generalizations as these sound- 

 ings give more and more detail. In most cases there is a relatively smooth bottom pro- 

 file in the broad extended deep-sea basins and considerably greater irregularity over 

 the central ridges and over the rises that separate the broad basins; considerable 

 elevations above the mean surface of an area occur frequently in the vicinity of great 

 depths and depressions so that extreme variations in depth are very often situated 

 close together. 



Only certain especially characteristic forms of the commonly occurring typical bot- 

 tom features will be discussed here. Stretching out to sea from the edge of the land 

 there is first the beach which at high water is part of the sea bottom and, at low water, 

 is part of the land. This amphibious part of the Earth's surface according to the 

 estimate of Schott has an area of 1-6 million km^ or about 0-4% of the ocean area. 

 Outside this the ledge-like rim appears, sometimes narrow, sometimes broad, but rarely 

 completely absent, and is called the continental shelf. From the boundary between the 

 land and the sea the sea bottom, except along coastal cliffs, slopes gently down at a 

 slight angle, at the most 1-1-5°, This angle gradually increases and near the 200 m 

 isobath it changes abruptly to the steeper gradient of the continental slope. The mean 

 slope angle is about 3° here but in isolated cases it may be appreciably larger (6-10° 

 or more). The edge of the shelf is normally at a depth of between 100 m and 200 m, 

 but in some cases it appears only at a depth of 400-500 m. The continental shelf is 

 seldom a uniform surface. It is very frequently broken by canyons, furrows and 

 troughs, and shows clearly the effects of the more intense movements of the water 

 because of the shallow depth (ocean and tidal currents). These effects of the action of 

 the ocean are not found everywhere; in some places the sea bottom has clearly been 

 formed during the ice ages by glacial action and has the character of a drumlin 

 landscape as in the Irish Sea, for instance, between Ireland and Scotland. 



The continental shelf can usually be regarded as a part of the continental block 

 which has been flooded by the sea, and its formation and topography are partly the 

 product of the separation of the continents through accumulation and partly due to 

 the erosion of the coast by wave action (Penck, 1934). Up to the present time no 

 detailed investigation of the extent of the continental shelf has been made. Usually 

 the 200 m isobath is taken as the outer limit of the shelf and the area of the shelf 

 within this is usually designated as "bathymetric". In his statistics of the ocean depth 

 Kossinna has listed these areas for each continent (Table 6). The bathymetric shelf 

 extends over an area of 27-5 milhon km^ or 7-6% of the area of the ocean; Wagner 

 has given the value 30-6 and Kegel has given 29-5 million km^. The mean depth of the 



