Organogenic (biochemical) rocks belong to the group of sedimentary rocks. They are divided into two subgroups: zoogenic and phytogenic. Tufa (bigor) is a very porous phytogenic rock that forms around the stems of plants in streams and waterfalls, the waters of which have a high content of calcium carbonate. Calcium carbonate is very poorly soluble in water. However, in the presence of carbon dioxide in the water, calcium dihydrogen carbonate Ca(HCO3)2 is formed, which is much more soluble. In this way, calcium carbonate (limestone) is washed out of the rocks in places and increases the hardness of the water. When the carbon dioxide content is reduced in some way, the limestone crystallizes again. This is how stalactites and stalagmites in caves are formed, for example, and also boiler scale. Calcium carbonate is white or colorless, but natural carbonate sedimentary rocks are colored in gray, brown, shades of pink, etc. due to the presence of impurities from other minerals. In the past, tufa was used as a material for cladding buildings (e.g. churches) and making various decorative elements (cornices, votive crosses, etc.) and less often as a material for walls. It was valued for its light weight and easy processing, but at the same time this stone has low strength, absorbs water easily and is not resistant to weathering. It was quarried in the area of the village of Pavolche. Now tufa quarring is prohibited because the karst springs where this rock is formed are protected habitats.
It is located at the springs of the Zlatitsa River. It is a cascade of several consecutive falls. The highest is about 5-6m. The waterfall is full-flowing in spring, and later in summer it dries up. Tufa barriers in the river gives the waterfall its name (bigor = tufa). The stones are overgrown with moss and fern and are very reminiscent of the Krushuna Waterfalls area in miniature. In the past the majority of the population of Zverino drew water for drinking, washing and laundry from the Zlatitsa River in winter. They drew water early in the morning for drinking and washing, and for washing in any weather. For washing, they also drew water from the Iskar River, which at that time (before the opening of the ore enrichment factory in near-by Eliseyna village) was still relatively clean and clear. During the years 1935-1938, with the voluntary work of local people, a water supply line was laid from Belio Kladenets (The white well) to the Slatina area (the springs of the Slatinska River, a right tributary of the Zlatitsa). The entire route was dug by hand. In 1968-1970, a water supply was laid from the Bigora waterfall, capturing the spring in the area with the highest flow rate.
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