To a geologist, gravel is any loose rock that is larger than 2 millimetres in its longest dimension but no more than about 60 millimetres. Gravel is simply rock that is within this specific particle size range. Larger rocks are gradually eroded into smaller rocks and this leads to the formation of natural gravel deposits. However, gravel can also be made by mechanically crushing rocks. Many roads and paths have a gravel surface, especially in low-traffic rural areas. It may also be used as an aggregate in concrete. There are more roads in the world that have a gravel surface than a concrete or tarmac one. There are more than 400,000 kilometres of gravel-surfaced roads in Russia. Rock particles smaller than gravel are geologically classified as sand. Rock particles larger than gravel and pebble are geologically classified as cobble. The erosion caused by rivers and waves tends to pile up gravel in large quantities. This can lead to the gravel becoming compacted into sedimentary rock, which geologists call conglomerate. Quarries where gravel is extracted are called gravel pits. There are many types of gravel. They include: bank gravel, which is naturally deposited gravel adjacent to rivers and streams and which is also known as bank run or river run; bench gravel, which is a layer of gravel on the side of a valley left over from when the stream lower down the valley was at a higher level; crushed stone, which refers to gravel that has been crushed and mixed to a blend of stones which, when used on roads and driveways, sometimes has tar imposed on it; fine gravel, which is gravel that is only 2 millimetres to 4 millimetres in its longest dimension; lag gravel, which is a surface accumulation of coarse gravel left behind after the removal of fine gravel; and pea gravel, which is a type of gravel consisting of small rounded stones used in concrete surfaces.