After the valley's bottom was formed, the stream of the Iskar River was blocked and the whole valley became a lake, causing the gritty and clay-like soils that now cover most of the valley.
Following the formation of the valley's basin, the flow of the Iskar River was halted, resulting in the entire valley being transformed into a lake, which subsequently led to the creation of the rough and earthy soils that currently cover most of the valley.