THE SEA 305 



upon the seaward portion of the platform, or upon a lower level 

 of previous formation. Further, this sediment will show by its 

 character that it actually was derived from the material cut away 

 by smoothing the plain, and the whole of it, even its bottom 

 layers, will be of marine origin. In such a plain the advancing 

 sea must have obliterated the stream valleys which had been 

 excavated when the region was land. This obliteration will be 

 performed partly by shaving down the divides, or watersheds, 

 between the streams and partly by filling up the valleys with 

 sediment. 



When the region is once more uplifted above the level of the 

 sea, an entirely new system of drainage will be established upon 

 it, determined by the slopes of the overlying cover of newly de- 

 posited sediments, and having no reference to the structure and 

 arrangement of the underlying older rocks. These newly estab- 

 lished streams may, if the upheaval of the country gives them 

 sufficient fall, cut down through the newer sediments. Indeed, 

 the latter may eventually be swept away entirely by the various 

 subaerial agencies, but the stream courses, which were determined 

 originally by the slopes of that newer sediment, will show little or 

 no adjustment to the structure of the underlying older rocks. 



These criteria are useful in identifying those plains which were 

 smoothed by the action of the sea ; but when the processes of sub- 

 aerial denudation have completely dissected the elevated area, all 

 such evidences may be removed and the origin of the plain may 

 become quite indeterminable. 



A coast-line newly formed by the elevation of land may be dis- 

 tinguished from one which has stood long at the same level by its 

 unworked character, the first effect of elevation being to produce 

 an even, regular shore-line. This is because the combined effects 

 of erosion and sedimentation tend to make the sea-floor flat and 

 smooth, and an elevation of such a floor to a given altitude must 

 produce an even and regular shore. Nearly the whole west coast of 

 South America is an example of this. On the other hand, a coast 

 which has long stood at the same relative level will show plainly 

 the long-continued action of the sea upon it. What results that 



