Vol. 68.] GEOLOGY OF COSTA EICA. 113 



lY. Caetago aitd the San Jose Valley. 



The drainage-system of the llio Grande and its tributary streams 

 stretches from El Alto, some 10 miles east of San Jose, almost due 

 westwards to the Pacific Ocean. The Cartago area drained by the 

 head-waters of the Rio Reventazon, and therefore belonging to 

 the Atlantic slope, presents, however, such close similarity to the 

 topography of the San Jose Valley that they are most conveniently 

 considered together. The topography is very striking, and shows 

 much the same features for a considerable distance on either side of 

 the watershed. A brief description of the San Jose Valley will 

 give some idea of the main points. On the south this valley is 

 bounded by the Cerro Candelaria, and on the north by the great 

 chain of recent volcanoes, Poas, Irazu, etc. Between these two 

 chains is a vast valley, which at San Jose cannot be less than 

 12 or 14 miles wide. It is remarkable for its extraordinarily 

 smooth flat surface ; it is, in fact, a great plain dipping gently to 

 the west as far as Atenas, where the Cerro Candelaria and the 

 Aguacate Hills converge, and between these the Eio Grande flows 

 in a magnificent gorge. On the south the mountains rise with 

 surprising abruptness from the floor of the valley, presenting almost 

 the appearance of a mountainous coast-line rising from the sea. 

 The northern boundary of the valley is very different ; the ground 

 rises at first almost imperceptibly from the valley, the slope 

 gradually increasing to culminate in the magnificent range of active 

 volcanoes. 



Between Alajuela and San Jose, however, this even grade is 

 broken by an abrupt escarpment 300 or 400 feet high, which forms 

 a great terrace along the lower slopes of the volcanoes. Various 

 writers ^ have regarded this and other similar terraces as due to the 

 former existence of large lakes, while Dr. Sapper ^ has put forward 

 a totally difi'erent theory to account for this feature. He regards 

 the San Jose Valley as having been partly filled up by material 

 coming from the recent volcanoes on the north, which spread south- 

 wards and was banked up against the Cerro Candelaria ; he supposes 

 that, during periods of volcanic quiescence, river-action has cut 

 down the general level of the surface, leaving a steep terrace on the 

 north side of the valley. He sums up by saying that it is the same 

 action on a very large scale as that which causes the formation of 

 river-terraces in alluvial deposits. This seems to involve river-erosion 

 on a scale altogether disproportionate to the possibihties of the case ; 

 and at present the efl'ect of river-action is not to lower appreciably the 

 general surface-level, but to cut deep vertical gorges with but little 

 evidence of lateral erosion. I was not able to examine this terrace 

 in any detail; but, from the general appearance, I -should like to 

 suggest the possibility of its being a fault-scarp. Considering the 

 immense amount of material which has been rejected by these 



^ a. Attwood, op. cit. p. 332 ; E. T. Hill, op. cit. pp. 225-26. 

 2 Zeitscbr. Deutscb. Geol. Gesallsch. vol. liii (1901) p. 28. 

 Q. J.G. S. No. 269. I 



