A boulder is bigger… than a pebble and the pebbles are bigger than gravel
For geologists, a piece of rock is never just a piece of rock. In geology, a measurement system, the Udden-Wentworth scale , is used to distinguish boulders, pebbles, gravel down to the tiniest grains of sand.
The size of the rocks, regardless of the material that composes them, is very important in many disciplines and is indicated by the technical name of granulometry .
The grain size is the property that identifies the individual particles that make up a sedimentary rock, soil or soil based on size. This property is a parameter used in Geology, Pedology, Agronomy, Geotechnics, Hydrology.
The particle size classification is independent of the chemical or chemical-mineralogical nature of the particles and takes into consideration only their size. The particles are divided into granulometric classes, defined differently according to the classification system used; in general, regardless of the dimensional parameters adopted by the different classification systems, the main grain size classes are usually 4, in descending order of size:
- gravel
- sand
- silt
- clay
Mainly two systems of granulometric classification can be distinguished, the Wentworh or Udden-Wentworth scale and the Krumbein scale . We are interested in the first one which is simpler and more suitable for us nature lovers.
| SIZE RANGE (METRIC) | GRAIN SIZE CLASSES (WENTWORTH) |
|---|---|
| > 256 mm | Boulders (Boulder) |
| 256 – 64 mm | Pebbles (Cobble) |
| 64 – 4 mm | Gravel (Pebble) |
| 4 – 2 mm | Very fine gravel (Granule) |
| 2 – 1 mm | Very coarse sand |
| 1 – 1/2 mm | Coarse sand |
| 1/2 – 1/4 mm | Medium sand |
| 1/4 – 1/8 mm | Fine sand grain |
| 1/8 – 1/16 mm | Very fine sand grain |
| 1/16 – 1/256 mm | Limo (Silt) |
| < 1/256 mm | Clay (Clay particle) |
A little secret to familiarizing yourself with size
For boulders and pebbles it is easy to distinguish them, but it can become difficult to distinguish gravel from other smaller rocks with the naked eye for those unfamiliar with measures or for children. A small graphic comes to our aid that can be printed and allows you to compare a real stone with the drawings on this graphic to recognize them based on size.
It can also be a game for children to become familiar with nature and the land that surrounds us.
Advice for the hiker
For us walkers and nature lovers, knowing the size and type of terrain is not just a curiosity but a very important technical detail that allows us to prepare ourselves in the best possible way.
From the technical information of a route, path or territory we can best prepare ourselves to understand very important information. Based on the type of terrain, you need to choose the right equipment to take with us.
Based on the type of terrain we can choose the right shoes to use also according to the season. Just think of a sandy ground in a rainy season, we already know that we will have to face a lot of mud, and in the case of summer it can be very dusty ground. Another example is a gravel ground, with high slopes it can be very slippery and difficult to climb, or a ground with pebbles always with high slopes can be very difficult downhill and can cause some pain and discomfort for those suffering from the knees. Knowing the terrain allows you to better prepare yourself to be able to leave.
References:
Wikipedia: https://it.wikipedia.org/wiki/Granulometria_(geologia)
geologyistheway.com: https://geologyistheway.com/it/sedimentary/granulometria/
