Cordillera Real (Bolivia) の氷河

抄録

The Cordillera Real, divide between the Altiplano and Yungas drainage basin, is located to the east of the Bolivian capital, La Paz. This high range has experienced violent glaciations in three times, which are denominated by the author as Pata Antigua, Pata Nueva and Milluni glacial age. Many glaciers yet remains around high peaks in the central parts of the range. Post-Milluni stage is divided into six sub-stages (M1, M2, …… and M6). All cirques now free from ice have been formed in the latest glacial age, Milluni.
In this paper the author analyzes by statistical method some factors related to the altitudinal distribution of glacial phenomena.
90 glaciers and 79 cirques analyzed in this paper are located in the National Map of Bolivia, sheet No. 5945 IV, Lago Khara Kkota, in scale of 1 : 50, 000, which covers the central parts of the Cordillera Real. Figure II shows the distribution of the above glaciers and cirques. Altitude was read from contour-lines, and area was measured with a polar planimeter in the original map of Figure II. Data were listed up in Tables II-1 and II-2.
Correlation coefficients between the altitudinal distribution of glacial phenomena and some factors shown in Tables IV-1 to IV-7 were examined by the method of the t-test of significance. The conclusions are as follows.
1) The large-sized glaciers are found around the higher summits, and small ones around the lower summits.
2) The ice-tongues of large-sized glaciers reach down to low altitude.
3) Neverthless it cannot be concluded that the glaciers around the high summits would reach down to low altitude.
4) The distance of retreat of the ice-tongues after the sub-stage M6 is proportional to the size of the glaciers. From this result, it may be concluded that the ice-tongues of large-sized glaciers are more sensitive to the fluctuation of climatic circumstances.
5) Altitude of the cirque-bottoms becomes higher in proportion to the height of summits around them. A great number of the cirques are located by 150 m to 550 m lower than the summits. It is inferred that the cirques were not formed concordantly with the level of snow line. A lower limit of the altitudinal distribution of the cirque-bottoms should be considered as the altitude of ancient snow line.
The conclusions of the t-test of mean values of the altitude of glacial phenomena are as follows.
1) Glacial phenomena reach lower in the southern than in the northern slopes. This is probably caused by the fact that the southern slopes are hindered from solar radiation by heavy clouds in the summer when the southern slopes should normally receive more solar radiation.
2) Glacial phenomena are found higher in the Altiplano than in the Yungas drainage basin. In is because the easterly snow-bearing winds are checked against the barrier of the Cordillera Real.