That was probably the only soil in the entire country! The mud that did the sliding in that mudslide.
Ive heard that the "whole country of Norway is just one big rock in the ocean" because the Ice Age glaciers just carried all of the topsoil away. And thats why Nobel had to invent dynamite. Before Nobel it took dozens of men to chisel a hole big enough for a house foundation, or even for a grave. After they could just light dynamite.