WOODWORTH: geological expedition to BRA.ZIL AND CHILE. 131 



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Malvoa Station, 79 kms., alt. 43 M. (141.0 ft.). Good terrace on 

 opposite or southwest side of river. At railway station there are clays. 



San Rosendo Station, So kms. alt. 46 M. (154.8 ft.). East of the 

 Station there was in 190S an excellent exposure of gravels and sands 

 in a deep cut, rising about sixty-five feet above the railway, or to an 

 elevation of 220 feet above sea-level. The annexed 

 section is from a drawing made on the spot. 

 (Fig. 37). 



This section reveals a succession of river deposits 

 varying from sands like those now transported in 

 this portion of its course by the Rio Bio Bio to 

 coarse gravels including large boulders such as 

 demand ice-rafts or the proximity upstream of 

 glacial conditions. 



The lowest bed exposed (see Fig. 37) consists 

 of dark sand with pebbles of a volcanic rock. 

 Above this comes a coarse gravel including a 

 boulder of gneiss about seven feet long. Next 

 in the section is sand with thin bands of volcanic 

 pebbles. Above these layers comes about twenty- 

 five feet of dark sands with a thin band of volcanic 

 pebbles. Surmounting this and forming the sur- 

 face is a layer, about twenty-feet thick, of coarsely 

 bedded gravels and boulders, the cross-bedding of 

 which suggests the structure of a delta front. 



There are in this section thus the records of two 

 episodes when the river transported to this point 

 coarse gravels and boulders which it is to be presumed indicate con- 

 temporary advances of the local glaciers or times of unusual melting 

 and discharge of coarse debris. 



I saw no marine shells in any part of this terrace or on its surface, 

 and no evidence of the presence of the sea in the deposition of the 

 materials. The structure is quite like that of many glacial river 

 deposits in the inland portions of glaciated North America. The 

 lower bed of coarse gravel with boulders must have been deposited 

 at or above sea-level as is also the case v.ith the uppermost bed of like 

 materials. The aggradation of the valley under glacial stream-action 

 affords a simple explanation of these deposits without recourse to the 

 hypothesis of a change of level in relation to the sea. 



This section is the most reliable in its bearing on the altitude of the 

 terraces, since those lower down the riAcr were estimated only from 



Cr 0<3 OO <70 



Fio. 37. — Section of 

 Pleistocene terrace 

 at San Rosendo, 

 Chile, in 1908. 



