482 PROOlfiEDINGS OF THE ALBANY MEETING 



the valley sides; but tlie slopes are more generally of rather even declivity, as if 

 pretty well graded. 



Marine abrasion as an alternative to sn})aerial erosion natnrally calls for consid- 

 eration in the origin of the uplands of Belle Isle, but it is of doubtful application. 

 There are no marine gravels on the uplands, and it is my impression that the 

 depth of upland soil is greater than could have been produced by the action 

 of weather on a sea- planed rock platform during no longer time than was suf- 

 ficient to erode the narrow valleys; but on tiiis point I have no direct evidence 

 to offer. On the other hand, the peneplain of Belle Isle is very probably of the 

 same origin as that of the mainland of Brittany, where uplands as even as those 

 of the outlying island are common. But here the larger area permits the occur- 

 rence of a greater variety of rocks, some of which rise in subdued ridges running 

 east and west througli the peninsula. Tlie prevalent occurrence of deep- weathered 

 local soils, the al)sence of all trace of sea cliffs along the baseof the subdued ridges 

 and of the many isolated hills associated with them, as well as the absence of 

 marine gravels along the coastal part of the uplands, where they descend slightly 

 toAvard the sea (marine deposits occur at lower levels), give strong evidence against 

 a marine origin of the plain, while an origin by bevelling or by stream spacing 

 seems here to find no room for consideration. 



80 strongly was this conclusion impressed upon me that the thinness of the soil 

 on certain marginal areas of the Brittany peneplain seemed explicable only by a 

 brief and local subnierg(Mice after subaerial peneplanation, thus allowing the waves 

 to remove most of the weak soil cover, Init not giving them time enough to cut a 

 shoreline. An example of this kind is given in plate -45, figure 2, taken from a low 

 hill (Mont Saint Michel — not the famous rocky island of that name) on the out- 

 skirts of the village of Carjiac near the southern coast. The occurrence of thou- 

 sands of huge standing stones in the neiglil)orhood of Carnac seems to show that 

 the ancient peoples here took advantage of the partial removal of the soil and of 

 the revelation of countless weathered and loos(Mied ledges in selecting this district 

 for their lenuirkable monuments. The peneplain is really not so smooth here as 

 in many other districts, however level it looks in the view. It stands close to sea- 

 level and dips under water gently, without being cliffed ; an excej)tional feature 

 for Brittany. The plain rises gently toward the interior, so that the streams in- 

 trench themselves beneath the upland surface and erode valleys whose depth is 

 at a maximum somewhat above the middle of their length. 



If arguments are to be advanced against peneplanation, they would seem to me 

 more convincing if land forms of the kind here presented should be considered 

 along with those of more mature dissection ; and if several processes are held to 

 be capable of producing even-toi)i)ed uplands, sharply trenched by narrow valleys, 

 then some means of distinguishing among the forms resulting from the action of 

 the several processes should be devised. • Furthermore, inasmuch as it is eminently 

 possil)le that peneplanation, followed by uplift and mature dissection, maj' pro- 

 duce forms that wjll closely imitate those attributed to bevelling and to stream 

 spacing, it is desirable to find particular signs by which the results of the various 

 processes may be discriminated in the later stages of erosion, as well as in tlie 

 earlier stages above illustrated. It is premature to appeal today to simplicity of 

 process or inferred probability of occurrence in favor of this theory or of that when 

 one is trying to explain land forms that may also be accounted for by any one of 

 several other theories. 8imj)licity is a strong recommendation, especially in artifi- 



