Chalk is a relatively soft rock that may be eroded or weathered in a number of different ways. It is porous, absorbing up to 20% of its dry weight in water, and is therefore highly susceptible to weathering by freeze-thaw action, which may occur over repeated annual or diurnal cycles. This frost weathering produces a mix of rubble and viscous mud, which may be washed downhill, further eroding the landscape (a process known as solifluction). Since chalk contains a very high percentage of calcium carbonate, it can be dissolved by both groundwater and rainwater.