This means that it allows water to seep through it easily. This is not the same as being porous, though the terms are often confused. Porosity is the capacity of a rock to hold water in spaces in the rock - in other words, how full of holes it is. A rock like slate is barely 1 per cent porous; gravel is over 30 per cent porous. A rock that is porous is likely to be permeable -
though not always - but a rock that is permeable is not necessarily porous.