While the Scandinavian Alps gradually increase altitude, erosion can negate this slow growth. While this includes river erosion, the glacial erosion, especially during the great glaciations of the Quaternary (from 1.5 Ma) has had the most influence on the current geomorphology of the chain and the reserve. Glaciers grew and invaded the valleys, and then gradually unified to form a continuous ice sheet that completely covered the area. The nature of the rocks had a considerable influence on amount of glacial erosion. Most of soft rocks of Koli layer were quickly eroded while the rocks of the Seve layer were more resistant to erosion. Thus, the peaks of Stor-Aigert, as well as the Ammarfjället, Rerrogaise, Sulåive and Stubebakte mountain ranges currently consist of amphibolites and have steep reliefs. Similarly, the highest points of the Artfjället and Norra Storfjället mountain ranges consist largely of gabbros. The general direction of water flow also affects the relief: the layers tend to rise from west to east, which implies that the west-facing slopes are gentle in general, while the eastern slopes are often steep. The most visible signs of the glacial morphology are glacial valleys and U-valleys, such as Syterskalet, the top of the Vindelälven Valley, Skebleskalet, etc. There are also some forty small glacial craters or niches, most facing east, on the leeward side.