Waterfalls usually distinguish young landscapes in which rivers have not yet managed to wipe out the obstacles in their courses. The great number of waterfalls in Iceland may partly be explained by the youth of the country in terms of geological history. Geologists divide waterfalls into two main categories. On the one hand, waterfalls which form because of river erosion due to varying bedrock consistency; and on the other hand, waterfalls which form because of faults, crack formation, wave and glacial erosion, and various kinds of damming. The Öxarárfoss waterfall is a typical waterfall of the fault type. Skeifárfoss, at Tjörnes, is one of these. Hjálparfoss is one example of such, falling from one of the many Þjórsá lava fields. The waterfall Glymur, in Hvalfjörðurs Botnsá river, is Icelands highest waterfall at 190 m. The beautiful waterfall Faxi is on the Tungufljót river, which originates in the heights above Haukadalur, formed by many spring-fed tributaries.