Washington Slagbaai National Park - GEOLOGY
Most of the older rock formations of Bonaire developed underwater, in times when Bonaire was not yet an island (90-100 million years ago). After the emergence of these formations (60-70 million years ago) two important events started to happen:
1. The rocks became exposed to erosion, the debris was carried off by
rain water and so hills, valleys and plains developed;
At Washington Slagbaai National Park both phenomena may be observed.
When visiting Bonaire, one can see two vastly different types of hard rock that tell us about the origin and subsequent history of our island. We observe dark rocks, either unstratified or thinly laminated, in the last case the strata are tilted, and dip 35 to 40 degrees down to the Northeast. Very different are yellowish or white limestone deposits in subhorizontal position. The geologists could prove that the dark rocks date back from the Cretaceous period, that ended 65 millions years ago. The light-coloured limestones are much younger, their formation started only about 5 million years ago.
Most rocks that we find in the Park have not been seen silently deposited as settling dust on the dark ocean floor. The tops of the Brandaris (240m.), Ceru'i Mangel (149 m.) and other hills consist of dark rock with white feldspar crystals. These rocks show a peculiar way of prismatic columnar jointing; there is no stratification at all. Such type of stones constitute the bulk of Bonaire's Cretaceous backbone. They certainly are ancient volcanic; chilled lavas, petrified tuffs. The rounded pillows in which lava flows are that are seen on Bonaire betray that there must have been a series of submarine volcanoes.
Younger strata are not tilted, we find them at most places along the coast and a close look tells us that these creamy-white limestones originated as reefs. We find branching coral and (chalky white) coralline algae that prove this contention. As nearly all organisms found petrified in these reefs are still existing, their age can not be great. Radio-carbon gives an age of 110.000 years. In those times the sea stood about 15 meters higher than it does now. The reef has been preserved as a terrace that fringes the island's coastlines. At some places even two or three terraces are present, since the sea has known very different levels, depending whether the polar ice caps retained more of less rainpour.
Most of the rivulets (temporary water streams) do not empty directly into the sea. Some empty into island bays or saliñas, salt lakes, the bottoms of which have sometimes partly dried up (e.g. Saliña Matijs and Boca Bartol). These bays are drowned valleys. When, in former times, the sea level was lower than at present, the rivers could erode their valleys to a lower level than the present sea level. When the water rose, the lower part of the valleys became submerged. Still later the mouth was blocked by walls of coral shingle put there by the surf. The well known Goto and Slagbaai owe their existence to such a process.
YOU ARE VERY WELCOME TO EXPLORE THE FASCINATING GEOLOGY INSIDE OUR NATIONAL PARK. HOWEVER, YOU ARE NOT ALLOWED TO REMOVE ANY MATERIALS (FOSSILS, ROCKS, SAND, ETC). THE SAME APPLIES FOR THE BONAIRE NATIONAL MARINE PARK.