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.
The formation of the valley's basin led to the cessation of the Iskar River's flow, which in turn resulted in the transformation of the entire valley into a lake, ultimately giving rise to the rough and earthy soils that currently dominate the landscape.