Regolith landslides, which involve the upper soil layer, occurred during the heavy rainfall that struck the Manawatū in February 2004. As water enters soil particles it increases their weight, and when the ground is fully saturated the water pressure begins to force the particles apart. This makes soil much more likely to slip. Compare the area of steep pasture with the scrub-covered gullies (upper right), where protective vegetation reduced the number of slips.