The rock formations in the Val Lumnezia are mostly composed of slate — geologically known as the Grisons slate belt — which forms the mountains and valleys south of the Vorderrhein. The slate is interbedded with dolomite and quartzite. The Greina high Alpine plain distinguishes itself from the remainder of the Val Lumnezia by the occurrence of crystallite rock formations, characteristic of the Adula Alps. As several tectonic faults have fractured the original rock formations, the geological origin of the Grisons slate is unknown, but it is probably sediment of the thickness of some , which has formed during the Jurassic Period in estuaries. There are two types of slate in the Val Lumnezia: North of the river Glogn the slate of the Adula Alps predominates, whereas South of the river Glogn the Lugnez slate can be found. (These strata run with an inclination of 75° towards the North West). The former has weathered substantially, the latter is more resistant. The most cohesive slate mass stretches across and area of approximately . South of this massive slate plate, again the rock formations show substantial tectonic disruptions.