The Plakánek valley located close to the medieval castle of Kost is one of the most interesting geological sites in Český ráj. The Klenice stream that flows through the valley today has cut through the sandstone block, revealing the different layers of sandstone sediment, laid down in the shallow Cretaceous sea that covered the area some 90 million years ago.

The Plakánek valley is one of the few locations in the region where the youngest layers of sediment are visible. These are mudstone layers, which have all been eroded away in the more central sandstone districts. The Plakánek valley therefore offers a visible timeline for the geological processes that occurred over a period of around 10 million years.

There is an interesting nature trail with dual language information boards along the valley.