Coarse Sand Beaches Research Articles

The purpose of this study is to obtain a better understanding of the forces acting on the rodent population in Venezuela. The site of the study is Zorro Island in Lake Valencia (Tacarigua). This locality was chosen because it is inhabited by a colony of Rattus rattus rattus, the size is adequate, there is little possibility of migrations, and meteorological data are readily available. The study began April, 1956; it still continues but the data presented cover only the period from that date to January 1958. Lake Valencia, in the North Central part of Venezuela is 33 km long, 18 km wide and has an average depth of 30 m. The level of the lake varies according to the season, but the average altitude of its surface is about 400 meters above sea level. There are 20 islands or protruding rocks., Zorro Island is the fifth largest island and is located in the southeastern part of the lake (100 09'N 67039'W) (Fig. 1). Its maximum dimensions are 450 m by 225 m, and its area is 6.5 hectares. The maximum height above lake level is 43 m. The shortest distance to other land areas, the Iguana Peninsula, is 750 m. Actually the distances are rough averages for the duration of the study, as the shore line changes due to the seasonal fluctuations of about a meter in the level of the lake's surface. Also, the general trend is for this level to decrease, having lost about 20 m since 1727. The island's origin has been traced to the Tertiary geological era. It was formed by projected masses of rocks and eruptive adjustments. The surface is composed of a steep central hill surrounded on the east, south, and west sides by a crescent-shaped, terraced, graduallysloping area that has emerged due to the decrease of the level of the lake. Based on this formation the island can be divided into four general areas: (a) shore, narrow strip of rocks with occasional coarse sand beaches; (b) emerged zone, largest component of the study sites; the basic soil is an accumulation of loam, humus, pebbles and small rocks that increase in size towards the central hill. The average slopes are: east side 30 percent, south 16 percent, west 23 percent. There is an unusual narrow vein of sandy loam traversing the southern part; (c) central hill, steep, 74 percent slope and formed by clusters of large rocks based over a rock bed; (d) hilltop, rather flat and with a surface similar to that of the emerged zone suggesting that the entire island was covered by water at some ancient period. Natural drainage of the island is excellent, and no surface water is found. The lake water is quite brackish with a pH value of 8.8

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