Variations in the water level, floods, marshes formation promote the growth of oak trees. Because of a continuous change of the direction of the river flow on a greater or lesser degree, the mainstreams weave through the vallies constantly forming live meanders. In its meandering course, the river undermines the banks covered with trees, which fall into the river and are swept away in the water. When the trunk gets trapped by its branches and roots in the river bed, over time layers of mud, sand and gravel cover it. Deprived of oxygen the wood undergoes the process of fossilization and a long process of morta formation.

During hundreds and thousands of years, under the influence of the minerals and iron from the water, the decomposition of oak timber is considerably slower. A special role is played by the currents of the underground waters in the creation of morta, binding its ingredients with larger quantities of the tannin in the wood and in this way darkening the wood. This centuries-long process, often termed “maturation”, turns the wood from golden-brown to completely black, while increasing its hardness to such a level that it can only be carved with the use of specially grind and exceptionally firm tools.

The time necessary for the oak to transform from the end of its biological growth to abonos varies. The “maturation” commonly lasts thousands of years. Due to the ecological reasons mentioned above no two trunks can be found of the same colour