The shape of stones


RANDOM SAMPLE: This sample shows the type of rock in the pebbles and their degree of roundness in one place along the river.

SMOOTH SHAPES: Rocks are seen in a mountain stream at Santubong. — Photo by Mary Margaret

Finding clues of the forces of nature

DURING a recent walk with my dog along a riverbank in the United Kingdom and while throwing stones in the river, I was reminded of the many hours I spent with Geography students surveying the bed of this river and that of the Danum River in Sabah.

Later, to avoid a field of sheep, I walked across a recently ploughed field, which was much higher than the river. However, the stones were of a different shape.

When planting shrubs or flowers in Kuching gardens, we occasionally come across a well-rounded pebble, yet most of the soils in the area are composed of silt, sand and clay, providing a deep veneer over the underlying bedrock.

The depth of the piling on construction sites bears witness to the depth of the soil over the bedrock. Why are these pebbles rounded?

As former river deposits of the Sarawak River, they have experienced abrasion and attrition as they ground against other pebbles on the former riverbed. These processes may be witnessed today at Damai Beach, where incoming and ebb tides roll the pebbles against each other, gradually rounding them and reducing any rough edges on the pebbles.

In 1963, at the University of Paris, French geographers Cailleux and Tricart produced an index of roundness (index of wear) for a pebble, that is easily applicable to any pebbles we may find. It is called the Cailleux Index of Roundness. Interestingly the French word ‘cailleux’ means pebbly!

To calculate the index of roundness of a pebble all one needs is a ruler, a semicircular chart and a calculator.

Take a pebble and lay its sharpest corner on the semicircular chart and assess the radius in millimetres (r) of the sharpest corner. Then measure its long axis (a) also in millimetres. Using these indices the Cailleux Index of Roundness (CI) can be calculated from the formula:

CI =(2r/a)1000

The nearer the CI approximates 1,000, the more rounded the pebble. This indicates that it has been well worked by water, for example, rivers or seas.

The roundness index will vary according to the degree of hardness of the geology of the pebble in question, the distance the pebble has been transported by running water, and the hardness of the other pebbles that it may have been in contact with during its transport.

A random sample from the bed of my local river shows the type of rock in the pebbles and their degree of roundness in one place along the river. Take a look at the photograph of the river stones and the calculations. This demonstrates the roundness of the pebble.

Pebbles tell us much about geological history and how their shapes are related to past and present forces of nature.

Dig a little deeper in your garden, find some pebbles and calculate their degree of roundness and their geological composition. However, avoid bits of brick and concrete blocks hidden under the soil by your house builder for they are likely to be very angular.

The size and shape of all stones depend initially upon the composition of the rock from which they originated and the subsequent weathering and transportation processes to which they have been subjected.

The key to the present always lies in the past.