Downstream of Bítouchov, the Jizera River transects a small massif of fractured albite granite, forming a canyon-like valley with up to 40 m high cliff faces and rock pillars. Upstream of the valley, slopes on the left ban of the Jizera River display different varieties of the Bítouchov alkalic-feldspar granite (aplitic facies, porphyroclastic, silicified, mylonitized). The narrowest tract of the valley is formed by strongly silicified metagranite (see Bítouchov site). Further downstream, the intensity of granite deformation increases until most of the minerals in the rock are crushed to form fine slurry. The rock then adopts the appearance of phyllite, and is therefore called phyllonite. Towards Spálov, belts of keratophyre alternate with belts of greenschist (metabasalt) and phyllite. In 1909, the valley was artificially widened and a trail for walking tourists was established here – called the Rieger’s Trail. A water power plant with an underground raceway and the maximum output of 2.4 MW was constructed in 1921–1926. The site is unique for the occurrence of Saxifraga caespitosa.