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| Hammen, T. van der; Wymstra, T.A.. |
| The pollen content of bore-hole samples and mine sections from the coast and from the bauxite belt of British Guiana has been studied. The pollen zonation is shown in fig. 6 and diagram IV. The description of the Upper Cretaceous and Tertiary pollen species is partly given in this article and partly in Van der Hammen, 1963; the Paleocene and Eocene species will be described in Leidelmeyer, 1965. The general picture obtained for the Guiana Basin, is summarized in three sections, one along the coast (fig. 18), one parallel to the Demerara River (fig. 24) and one parallel to the Berbice (fig. 25). The more detailed interpretation and correlation of the two deep coastal wells of Rose Hall and Shelter Belt is given in fig. 5. The situation in the bauxite areas... |
| Tipo: Article / Letter to the editor |
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| Ano: 1964 |
URL: http://www.repository.naturalis.nl/record/505816 |
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| Hammen, T. van der; Gonzalez, E.. |
| In a C 14 dated pollen diagram from ”Cienaga del Visitador” (ca 6°8’N; 72°47’ W) in the Colombian Eastern Cordillera the zones Ib + Ic + II (including the Allerød and Bølling interstadials) form one fluctuation in the diagram, as the short cold zone Ic is not reflected. An earlier Late-glacial interstadial is recognized and is called Susacá-interstadial. It probably lasted from about 13900 to 13100 B.P., was colder than the Bølling-interstadial, and is probably reflected in pollen diagrams from other parts of the world. The Holocene part of the diagram shows very high Gramineae-percentages, apparently due to a considerable lowering of the ”tree-line”. This must have been caused by the fact that the Holocene local climate has been much drier than the... |
| Tipo: Article / Letter to the editor |
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| Ano: 1965 |
URL: http://www.repository.naturalis.nl/record/505845 |
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| Hammen, T. van der; Gonzalez, E.. |
| 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... |
| Tipo: Article / Letter to the editor |
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| Ano: 1965 |
URL: http://www.repository.naturalis.nl/record/505957 |
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| Hammen, T. van der. |
| Pollen diagrams have been prepared of eight sections of Quaternary sediments from different localities on the coastal plain of British Guiana, and partly dated with the C 14 method. A Riss-Würm interglacial transgression, a Würm-glacial regression and a Holocene transgression have been established. The Würm-glacial vegetation on the place of the present coastal plain area was a poor grass-savanna type. The Holocene transgression at about 9500 B.P. is represented at 23 m. below present sea level and the maximum attained around 6500 B.P. when the relative sealevel was at least 2½ m. above that at present. |
| Tipo: Article / Letter to the editor |
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| Ano: 1963 |
URL: http://www.repository.naturalis.nl/record/505960 |
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| Wijmstra, T.A.; Hammen, T. van der. |
| A number of pollen diagrams from the Llanos Orientales of Colombia and the Rupununi Savannas of Guyana show that the actual open savannas were preceded by a closed savanna woodland or dry forest with Byrsonima as principal pollen producer. Human influence during the last 3000 years was apparently an important factor in the extension of open savannas. Nevertheless, open savannas occurred also earlier in the Holocene. Even in approximately Late-Glacial time, open savanna sometimes extended temporarily at the expense of the dry forest or woodland. It seems that climatic and atmospheric influences (periods of very dry climate, lightning) were the cause of these extensions. The pollen diagrams suggest anyhow a very unstable equilibrium between the woody... |
| Tipo: Article / Letter to the editor |
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| Ano: 1966 |
URL: http://www.repository.naturalis.nl/record/505833 |
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