Discrete Fracture in Rocks
Have you ever noticed how water can flow through rocks and leave a trace of its passage by covering the surface with a patina, like white stripes? The whole process is quite complex but can roughly be explained by breaking it down into two coupled effects: gravity and chemical reactions. Gravity causes the water to infiltrate through discrete fractures (hard rocks) or pores (sedimentary rocks), until it charges the groundwater supply below to a point where excess water will surface on the face of the rock, like an overflowing glass of water. Chemical reactions, on the other hand, allow water to dissolve carbonates and similar substances present in the rock. Such chemical compounds will start sticking and accumulating on the surface.