848 



PRINCIPLES OF STRATIGRAPHY 



creases, so that the process of base-levehng goes on at a progres- 

 sively diminishing rate. 



While the harder or more resistant strata of any region are the 

 last to be reduced to the level of the peneplain, they eventually also 

 succumb, and the surface of the peneplain thus shows a lack of 

 conformity to the structure of the country. This lack of conform- 

 ity to structure is one of the most characteristic features of a pene- 

 plain, and the one by which it is most readily recognized. When 

 the peneplain is gradually submerged beneath a transgressing sea, 

 the final inequalities may be smoothed off by marine planation. In 

 this manner erosion surfaces of remarkably level character may be 

 produced, such as are seen on the Archaean granites of the Manitou 



Fig. 221. Section on San Juan River, Colorado, showing erosion escarp- 

 ments in horizontal and tilted strata. 

 Fig. 222. The same section interpreted by the assumption of a fault. 



region in Colorado, where the early Paleeozoic sandstones rest upon 

 a surface almost as level as a table top. (Crosby-i.) (Fig. 52, p. 

 310.) Where transgression of the sea is gradual and uniform on a 

 peneplain surface, a basal conglomerate or sandstone is formed 

 which everywhere rests directly upon the old peneplaned surface. 

 The age of this sandstone or conglomerate will, however, vary as 

 pointed out in Chapter XVIII, being younger shoreward and older 

 seaward. Where monadnocks rise above the level of the submerged 

 peneplain, these will be gradually buried under the accumulating 

 coastal plain strata, which along their contact with the monadnock 

 will be of a more or less coarsely fragmental character. A typical 

 example of a monadnock buried in coastal plain strata, and now 

 partly resurrected by erosion, is found in the Baraboo ridges of 

 southern Wisconsin. An example of a monadnock being partly 

 buried by marine sediment is found in the island of Monhegan, off 



