Just some quick facts about pebbles.
A pebble is a clast of rock with a particle size of 2 to 64 millimetres based on the Krumbein phi scale of sedimentology. Pebbles are generally considered to be larger than granules (2 to 4 millimetres diameter) and smaller than cobbles (64 to 256 millimetres diameter).
A rock made predominantly of pebbles is termed a conglomerate as can be seen at Robberg in Plettenbergbay. A beach composed chiefly of surface pebbles is commonly termed a shingle beach. This type of beach has armoring characteristics with respect to wave erosion, as well as ecological niches which can provide habitat for many animals and plants.
Pebbles come in various colors and textures, and can have streaks of quartz and different colored sedimentary rock. Pebbles are mostly smooth but, dependent on how frequently they come in contact with the sea, they can have marks of contact with other rocks or other pebbles.
Pebbles are found in two locations – on the beaches of various oceans and seas, and inland where ancient seas used to cover the land. When then the seas retreated, the rocks became landlocked. They can also be found in lakes and ponds. Pebbles can also form in rivers, and travel into estuaries where the smoothing continues in the sea. Beach pebbles and river pebbles (also known as river rock) are distinct in their geological formation and appearance.
Beach Pebbles
Beach pebbles are formed gradually over time as the ocean water washes over loose rock particles. The result is a smooth, rounded appearance. The typical size range is from 2 mm to 50 mm. The colors range from translucent white to black, and include shades of yellow, brown, red and green.
Inland/fluvial Pebbles
Inland pebbles (river pebbles of river rock) are usually found along the shores of large rivers and lakes. These pebbles are formed as the flowing water washes over rock particles on the bottom and along the shores of the river. The smoothness and color of river pebbles depends on several factors, such as the composition of the soil of the river banks, the chemical characteristics of the water, and the speed of the current. Because river current is gentler than the ocean waves, river pebbles are usually not as smooth as beach pebbles. Stronger rivers will have smoother pebbles. The most common colors of river rock are black, grey, green, brown and white.
Shapes of Pebbles
The smoothness of a pebble is largely dependent on the length of time a pebble has been shaped as well as the composition of the rock, the chemical characteristics of the water and the amount of friction received from its surroundings. The shape and type of rock from the original fragment is said to play a major role in the shape of pebbles.
There is much confusion in geologic literature as to the shapes of fluvial and beach pebbles and the differences between them, if differences exist. Though the contrary has been asserted, most geologists who have written on the subject appear to hold the view that beach pebbles are generally flatter than river pebbles, having discoid, lozenge-shaped, ellipsoid, or oval forms.
It is asserted by some that these forms are produced by pushing of the rock fragments to and fro by the waves. Others have considered that the shapes of the original fragments and the inherent structure of the rock are dominant in determining the shapes of beach pebbles. That beach pebbles, even those composed of massive igneous rocks are commonly of a flattened oval form seems certain, as has been stated elsewhere, but this fact is probably to be attributed to the development of such forms from original flat fragments or from rocks of schistose structure or to the segregation of such forms under the peculiar action of the waves, rather than to their production by a specialized wave abrasion.
Logging this Earth Cache:
To log this cache you have to answer the following questions and submit them by contacting me via my profile here: http://www.geocaching.com/profile/?guid=725aed9a-cab8-4487-88b1-e57698b634a8.
1) Describe the pebbles on the beach at ground zero referring to their color, texture and size?
2) How where these pebbles formed?
3) What is said to be the main difference between beach and inland pebbles?
Sources: http://en.wikipedia.org/wiki/Pebble; http://pubs.usgs.gov/pp/0131c/report.pdf and coalminingandgeology.com