After the karst, limestone rock rose up out of the ancient sea, flattening the surrounding lands and cutting a deep canyon, the Krka River waterfalls were formed, thus creating the present day appearance of the river bed, and the characteristics of the living world. The waterfalls are the youngest formations in the canyon.
The travertine building process began in the Pleistocene, and has continued to the present day, with some interruptions. At the end of the last glacial age called Würm, the climate became warmer, water flow slowed and the conditions were created for the colonisation of algae, mosses and other organisms that take part in the travertine building process. In the Krka River Valley and its tributaries, the Pleistocene travertine is found up to 20 metres higher than today’s riverbed, which tells us of the level at which the water flowed during that time. This travertine is called fossil travertine.
The majority of the travertine at the waterfalls is less than 10,000 years old. With the creation of travertine deposits in the post-Würm period, Skradinski buk, Roški slap and the other waterfalls began to rise up above the water level, creating Visovac Lake and other accumulations in the canyon sections of the present-day Krka River.
The travertine forms differ in their age. The youngest travertine formations are the thresholds that form under the water surface. There are also barriers, travertine curtains, cones and many other forms. In terms of the structure and size of travertine barriers, we know that the most travertine is deposited near the end of the river course, where the water flow rate was slower. Skradinski buk waterfall is the longest travertine barrier on the Krka River and in Europe.
Travertine is also called tufa. This is the name for calcium carbonate (limestone) that settles out of running waters onto various types of substrates. Travertine will settle only in waters that have sufficient quantities of dissolved calcium bicarbonate. The waters in karst are rich in this mineral, as karst landscapes are made up of limestone and associate rock, which together are called carbonate rocks. They are easily dissolved and create very strange forms, both on the surface and underground. In order for these chemical processes to unfold unhindered, temperature, water flow rate, pH value of the water, dissolved oxygen concentrations and the organic matter content in the water are also important variables.
The travertine building process requires travertine building organisms, which are aquatic algae and mosses upon which these minerals settle. Without them, the calcium carbonate would not be retained, as it would be washed downstream, especially in places where the water flows quickly, mostly at sites with an uneven river bottom. The slope and structure of the riverbed are not uniform at any of the waterfalls, meaning that today we can enjoy the uniqueness of each.
The travertine growth conditions are most favourable in the present day, and this process can be seen along the entire course of the river, with new travertine deposits being continually formed on the Krka River, which make new waterfalls. However, it should be stressed that travertine is highly sensitive to water pollution and increase concentrations of organic matter in the water. Therefore, our primary function is to preserve the travertine barriers as vulnerable parts of the natural system, and to ensure the water and surroundings areas are clean.
For centuries prior to the proclamation of Krka National Park, travertine (tufa) was used as a building material in the areas along the Krka River.
Even the Church of St. Nicholas at Skradinski buk waterfall was constructed entirely of travertine. Fossil travertine is still visible today in Šibenik, along the staircase under the monument to King Petar Krešimir IV, and the Robert Visiani Gardens. There are also living “pieces” of travertine to be found in Šibenik. Part of the travertine from the Krka River has been built into the fountain in front of the Church of Our Lady Outside the City. It was placed in a sunny and wet location, and has continued to grow. Today it is overgrown by mosses and is several times larger than it was when it was placed here. This is a rare and valuable specimen of the many ways of cohabitation of the people with the river, which was so important in the past. Since the proclamation of Krka National Park, the extraction of travertine is strictly prohibited, as this rock represents the fundamental phenomenon of the Park.
Travertine is an excellent building material, and was used extensively prior to the prohibition of its use. Due to its moisture content, it is easy to shape, and upon drying out, the rock becomes very hard, due to the secretion of calcium carbonate from the calcium bicarbonate contained in the moisture. It has a specific hollow spongy appearance, and is usually beige to yellowish in colour. Even the ancient Romans used travertine to construct their temples, waterworks, monuments, baths and amphitheatres. The Roman Colosseum is the world’s largest structure built of travertine rock. Travertine is still used in construction today, though most often in interior design, where it is used as stone slabs for lining terraces and garden paths. The travertine from the Krka River is not used.
Sand was also previously excavated from the Krka for use in construction. Sand pits still visible today are the St. Jera pit and the islets Veliki Busen and Mali Busen just upstream from Skradinski buk. The St. Jera pit contains a sand deposit, and is much smaller today than it was 50 years ago. The islet became smaller due to the previous extraction of sand for housing construction. Veliki Busen was one of the larger and more abundant sand “mines” and due to the extensive exploitation of this resource, it is now much smaller than Mali Busen.