There are broadly speaking two types of rock of interest -igneous and sedimentary. Igneous rocks have been formed by volcanic action and are common in mountainous regions. They are extremely hard and dissolve slowly, helped by the slight acidity of naturally occurring rain. Water draining from igneous rock will contain most of the common metals, such as magnesium, calcium, sodium and potassium, but usually only in small amounts. The degree or amount of rock weathering that occurs has two important effects on the character of the drainage water.
Water hardness is determined mainly by the amount of dissolved magnesium and calcium ions there are.
This has important consequences for the fish-keeper. When these ions are dissolved they tend to neutralise the acidity of rain. Sedimentary rocks, on the other hand, are built from a jumble of rocks and some organic debris. Chalk or limestone are common examples. These rocks usually dissolve fairly easily so drainage waters contain high concentrations of the major ions, such as calcium and magnesium.