On average, the park receives 18-23 cm (8-10 inches) of precipitation a year. Drops of rainwater soak into the porous Entrada sandstone easily and then slowly dissolve the calcite bonding the sand together. Water puddles just above the denser Carmel layer where it erodes a cavity. In winter, water trapped between the two layers expands when it freezes and pries the rock apart. If the park received too much precipitation, the sandstone could erode so quickly that arches might not have time to form.