The movement of water through limestone

The porosity and the permeability of limestone is important because these properties control how water moves over and through the rock. There are several possible routes that water might take:

  • between the grains themselves via pores in the rock
  • through cracks, joints or fissures
  • through caves

Porosity

Chalk is highly porous

The porosity of a rock is the proportion of the rock that is made of the spaces between the grains (known as pores), voids and cracks.

Some limestones, for example chalk, are very porous, but others, like the massive Carboniferous limestones, are not at all porous — although they are permeable.

In limestones there are three main types of porosity:

Primary porosity

Figure 1: ooidal limestone

Primary porosity refers to the gaps between particles that developed during the final stages of sedimentation or that were present within the sedimentary particles at the time of deposition. 

Spaces (pores) between the grains of calcite, shells, sand grains etc, and within the grains themselves (shells, corals, etc. have pores and holes within them) are called primary porosity.

Figure 1 shows an ooidal limestone seen under the microscope. This is a very porous limestone through which water (and oil and gas) can move quickly.

Secondary porosity

Figure 2: shelly limestone

Secondary porosity happens when holes are made in the rock after the limestone was formed. They are usually caused by dissolution by acidic water or due to changes in the crystals (like when calcite changes to dolomite).

Figure 2 shows a thin section of shelly limestone in which some of the ooids have dissolved, creating pores, after the limestone formed.

Reduced porosity

Chalk

The porosity in rocks can decrease after burial. Pores are closed when the rock is squeezed and compacted underground. Porosity is also decreased by the precipitation of silica or calcite in pore spaces from percolating fluids.

Figure 3 shows chalk. Chalk is normally a porous rock, but when squeezed the pores close, so that water cannot move through the rock easily.

Permeability

The permeability of a rock is its ability to allow fluids (e.g. water) to pass through it from one pore space to another by capillary action or along cracks and fissures. Water can pass freely through permeable rocks but have difficulty passing through impermeable (or impervious) rocks. Permeability is measured by the rate of flow of a fluid.

Permeability is associated with porosity of a rock because the ease with which a liquid moves through a rock by capillary action depends on the size of the pores and how well they are interconnected.

Devil's Dyke, West Sussex

Porous rocks allow water to soak in quickly, greatly reducing surface runoff, and so are resistant to erosion.

Chalk is highly permeable rock and produces a landscape in which surface water is absent, although dry valleys may occur.

Dry valleys are a characteristic feature of chalk areas, which do not generally support surface drainage. The valleys probably began to form during an ice age, when deep permafrost made the ground impermeable.

Impermeable rocks

Rocks formed by some minerals do not have pores or joints and are not permeable (impermeable). Clay minerals are very tiny, flat crystals stacked one on top of the other like playing cards so that pores large enough for water to move through do not form and permeability is very low. These may form thin beds in the limestone, which interrupt the flow of water and may form spring lines.

Permeability can also depend on how the rock was made. For example, if there was a lot of pressure on the rock (perhaps because the sediment was buried deep beneath the ground) the grains would be squeezed close together, reducing the pore spaces and therefore also permeability.

Mineralised water that moves through porous limestone precipitates minerals like silica and calcite so that the pores and cracks are filled and permeability is reduced.

Porous and permeable

Previous sections have noted that the movement of water over and through rocks depends on their permeability and porosity. We have also seen that massive Carboniferous limestones are not porous, but they are permeable.

Rain is able to flow over the surface of Carboniferous limestone for only a short distance before it disappears. This is because the limestone has a lot of joints, faults, bedding planes and caves, which allow water to move through them quickly; permeability is high.

We will take another look at the movement of water through limestone when we consider cave systems in a later topic.


Carboniferous limestone at Malham Cove

Level limestones

It is noticeable that on limestone pavements, like those around Malham Cove in Yorkshire, rain puddles form in the small hollows (karren) on horizontal limestone blocks (clints), and is lost through evaporation rather than soaking into the rock. This is an indication that Carboniferous limestone is not porous or permeable.


Carboniferous limestone below ground

Sloping limestones

However, on sloping limestone pavements, the rain water flows down into the joints in the limestone pavement (grykes) and disappears almost immediately. This is because the water follows joints and fissures down to the drainage system below ground.

Joints and fissures gradually widen because rain dissolves the limestone. As the joints widen, so the rate of flow increases.


'Disappearing' rivers

Few rivers or streams flow over limestone areas. Streams that occur in the Pennines flow over the grits, sandstones and shales, but as soon as they meet areas of limestone, river courses disappear into sinkholes and swallow holes (dolines) to leave dry valleys.

The water follows the dolines, joints, fissures and cave systems to move through the massive limestone, then re-emerges elsewhere. Spring lines and resurgence occurs where the limestone rests on more impermeable sandstone or shale, or at the water table.