Hard Limestone and Pits Influenced by Lichens
Hard limestone and dolomite in nature
The weathering of hard limestone in the desert as compared to its weathering in Jerusalem compelled me to study the phenomena dealt with in this extended chapter. When I started work in 1963 in the Department of Geobotany at the Negev Institute for Arid Zone Research, our first task was mapping the Negev vegetation. Following a few reconnaissance excursions, we concluded that hard limestone and dolomite rocks, when in north-facing slopes, are populated by special plants. Examples were: narcissus near Dimona (Fig. 6.6.1) and Sternbergia (Fig. 6.6.2) near Yerokham. Both are growing in soil pockets in outcrops of smooth-faced hard rocks. At more advanced stages of the verification it became clear that the rock outcrops covered with epilithic lichens ([useful_plants_f1 “1” in Fig. 6.1.6 and Fig. 6.1.9]) are relatively smooth.
When rain falls, run-off water starts to flow very soon. After further observation (Figs. 6.6.2, 6.6.3) it became clear that organismic activity was involved here. The basic hypothesis was that in a site covered with epilithic lichen ([useful_plants_f1 Fig. 6.1.9]), direct contact between the rain water drops and the rock is prevented. Each drop of water has to pass through the lichen before reaching the rock. As long as it is epilithic lichen the impact is identical. Thus, rock surface totally covered by epilithic lichens becomes smooth even if there are patches of soft rocks that are ready for fast weathering.
A very local situation is displayed in Fig. 6.6.3 where the part of the dolomite rock facing north is smooth, whereas the part of the same rock facing south is pitted. When visiting the rocky region of my childhood in Jerusalem, I checked the dolomite rocks near Bet-HaKerem (there were once such rocks in the area between Bet HaKerem and Shaarei-Tzedek Hospital, that is now totally built up). The north-facing side was smooth, as in the Negev, but the south-facing side of the rock blocks looked like a jigsaw puzzle ([useful_plantsf7 Fig. 6.7.5]). Later I used these three weathering patterns – smooth, pitted, and jigsaw puzzle-like patterns, to divide the country into bioclimatic zones and to reconstruct climatic changes in the past.
While epilithic lichens cover the north-facing side of the rock (Figs. 6.6.3, 6.6.4), the south-facing slope, known to be drier in our latitudes, is pitted. What is the meaning or significance of the pitting?! Fig. 6.6.5, photographed near the Dead Sea, south of Enot Zukim (‘Ain Fashkha’) displays a high, white surface with pits that join here and there to form a channel. There are places where a white line is seen in the dark-colored channel floor. This is a “vein” in the rock where the organisms started populating the rock. Enlarging the scene area by Scanning Electron Microscope (SEM) shows (Fig. 6.6.6) that even the higher and white surface is sparsely populated by organisms situated in minute craters. Enlarging such a crater (Fig. 6.6.7) displays the thali (body) of the fungi of the genus Lichenothelia. Each thali dissolved the rock near it and the fungus is therefore in a depression. This species propagates by spores (Fig. 6.6.8), but also by branches with a swollen tip. In a similar rock surface near Sede Boqer another species of Lichenothelia (Fig. 6.6.9) has minute balls on the surface of its thallus; they might function in propagation as well.The transition from the mini-landscape where Lichenothelia prevails, to the mini-landscape of cyanobacteria and cyanophilous lichen (mentioned in the walls of Jerusalem and the valley of Kidron), is sharp. Its significance is that the rate of weathering near the Lichenothelia is much slower (Fig. 6.6.11) and the holes in it contain: 1. Cyanophilous lichen, and 2. Non-lichenized cyanobacteria. As a result of the release of much CO2 into the water in the pitted area, the dissolving ability of the water in that area is high and many limestone crystals become free (“3” in Fig. 6.6.11) and ready to be splattered by the rain drops. Since the organisms where the weathering rate is faster, expand at a similar rate in all directions, the population looks circular and the oldest place is also the deepest – thus pits are formed.
These microorganisms are genuinely endolithic as they dissolve their way into the rock (“2” in Fig. 6.6.12). The organisms create holes in the rock and swell when wetted, which brings them into direct contact with the walls of the holes, accelerating their dissolving (“1” in Fig. 6.6.12). As the years pass, the pits become denser in all the area that is suitable for their growth. A rock from near Kfar Adumim (Fig. 6.6.13) displays a brown-red layer of chert that is not weathered by pitting; however, the limestone below it is densely pitted and occasionally influences the deterioration of the chert as well.