A few words about the geology of the upper valley:
The upper part of the valley starts below the eastern face of Tolsta Košuta / Dicke Koschuta. Mountain's very upper part is built of the Late Triassic "Dachstein Limestone", below which is a bit younger bedded limestone. But below the altitude of some 1800 m, there's the area of very brittle Mid Triassic "Schlern Dolomite", which forms bizarre towers, crags and decays in broad areas of rubble. This is also the area of conglomerate and breccia, the rocks also prone to landslides. So, down in the valley-end we find all these rocks, which were in thousands of years transported down by landsliders, avalanches and water.
The most picturesque are colorful boulders of Tarvisio Breccia (see pictures). This mostly red paleozoic (Permian) rock has nice grains of white limestone, quartz, and other rocks. All those boulders came down to the creek from landslides higher up on the eastern face of Tolsta Košuta. Then by the creek there are nicely folded layers of Triassic limestones and marls (picture), then black plates of slate (picture) and many other types of rocks. All these are surrounded by light grey towers and crags of brittle Middle Triassic Schlern Dolomite (picture).
Rockfall is the only substantial objective danger. When in the narrow gorge, be careful if anything moves on the slopes above you!
Plan enough time, because on rocky, pathless terrain you will move slowly and carefully.