The various names given to small rocks and particles are based on a system of classification that reflects their size and resulting behavior in nature. These materials are technically known as clastic sediment, which are fragmented pieces of pre-existing rock that have been broken down through weathering and erosion. The size of these fragments, or clasts, determines where they can be transported, how they are deposited, and what kind of rock they will ultimately form when solidified.
Common Names for Small Rocks
The largest class of small rock fragments is broadly termed gravel, which is a collective category for particles larger than two millimeters in diameter. Within this category, most people are familiar with the term pebble, which describes a rock fragment that is small enough to be easily picked up but too large to be classified as sand. Pebbles are typically rounded due to long transport by water, and they range in size from about four millimeters to 64 millimeters.
Cobbles are the next size step up from pebbles, defined as rock fragments larger than 64 millimeters, which is roughly the size of a tennis ball, but smaller than the maximum size of 256 millimeters. Cobbles are generally too large to be moved by a strong stream current, requiring high-energy environments like a rapidly flowing river or a strong wave zone to transport them. A rounded, smooth pebble suggests significant wear and transport, while a cobble may still retain some angular edges if its journey was shorter.
The Science of Size Classification
Geologists require a more precise and standardized method than common names to classify these fragmented materials, which is provided by the Udden-Wentworth scale. This system uses specific diameter measurements in millimeters to define the exact boundaries between sediment categories. The scale is logarithmic, meaning each size class is twice as large as the next smaller one, allowing for consistent mathematical analysis of sediment size distribution.
For example, the boundary between gravel and sand is precisely set at two millimeters, while the split between sand and silt occurs at 1/16 of a millimeter, or 0.0625 mm. This exactness is necessary for a process called “sorting,” where a geologist determines the uniformity of a sediment sample. Poorly sorted sediment contains a wide variety of sizes, which can indicate rapid deposition, while well-sorted material suggests a more consistent transport agent, such as wind or waves.
This technical classification is important for engineering projects, such as determining the stability of a foundation or predicting how water will flow through a layer of sediment. The Wentworth scale provides the necessary precision for these applications.
Particles Smaller Than Sand
The finest end of the sediment spectrum includes particles smaller than the 0.0625 mm limit. Silt represents the coarser portion of this fine material, with individual grains ranging from 0.004 mm to 0.0625 mm. Silt particles are still composed of mineral fragments, primarily quartz and feldspar, and feel gritty when rubbed between the fingers.
Clay particles are the smallest, defined as having a diameter less than 0.004 mm, though the most commonly used boundary is 0.002 mm. Unlike silt, clay is not only defined by its minute size but also by its unique chemical and physical structure, which is typically a sheet-like lattice of silica and alumina. This plate-like shape and extremely high surface area give clay its distinctive property of plasticity, meaning it can be molded when wet and becomes hard when dried.
When saturated with water, silt and clay combine to form mud, which exhibits different behaviors than sand or gravel. Silt-rich mud tends to be less cohesive and more permeable, allowing water to pass through more easily. Clay-rich mud, however, is highly cohesive due to the electrostatic forces between the tiny plates, allowing it to retain significant amounts of water and making it virtually impermeable.