| Resumo: |
The Laguna de La Herrera (alt. ca 2550 m) is a lake situated on the western border of the Sabana de Bogotá, near Mosquera (dept. of Cundinamarca, Colombia) (fig. 2). This part of the Sabana has a relatively dry climate (appr. 600—700 mm rainfall), as it lies in the rain-shadow of the hills that border the Sabana on its western edge, and it bears therefore a xerophytic vegetation. The western slopes of the bordering mountains, that fall steeply to warmer valleys, have a much higher rainfall and are almost continuously clouded. They bear therefore a cloud-forest, of the Quercetum type, that reaches partly the very top of these mountains. Fig. 1 shows this in an idealized section. For further details on the mentioned vegetation-types, we may refer to van der Hammen & Gonzalez (1960). In the same publication the Geological history of the Sabana is shortly described, including the Quaternary history as a big lake with fluctuating water-level. Geological data on the area of Laguna de La Herrera were given in van der Hammen & Parada (1958). The present section of the pollen diagram corresponds approximately to bore-hole no. 19 of fig. 2 of that publication. The origin of the lake is probably (at least partly) due to fluvial erosion and sedimentation (old course of Rio Balsillas?). The lake sediments consist principally of diatom gyttja with intercalated layers of clay or peaty material. The base consists of hard greenish to white clay. We believe that this clay possibly corresponds to the altered clays that are exposed in the nearby hills, and which belong to much older eroded lake-sediments (see van der Hammen & Parada, 1958). The only other existing pollen diagram from the Holocene of the Sabana is from near Bogotá, near the eastern border of the Sabana (section CUX upper part, fig. 7 of van der Hammen & Gonzalez, 1960). Nevertheless, that diagram shows a completely different picture, reflecting a local vegetation (it is not from lake-sediments), under much more humid conditions (alternation of Alnetum and Myricetum). A direct comparison of the diagrams of Bogotá and La Herrera is therefore difficult.
|
