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Gravel is a loose aggregation of rock fragments, mineral grains, or shell fragments, larger than sand (2 mm) but smaller than cobbles (64 mm). It is an unconsolidated sedimentary material. The composition of gravel is highly variable, reflecting the geology of its source area. Common constituents include quartz, feldspar, chert, and various rock fragments (e.g., granite, basalt, limestone, sandstone). The clasts can be angular, sub-angular, sub-rounded, or well-rounded, with rounding increasing with transport distance and energy of the depositional environment. It is typically poorly sorted to well-sorted, depending on the depositional process.
How to Identify
- Color
- Highly variable, depending on the constituent rock and mineral fragments. Can be white, gray, black, brown, red, yellow, or a mix of colors.
- Luster
- Variable, depending on the constituent minerals and rock fragments. Can range from dull to vitreous, earthy, or greasy.
- Texture
- Clastic, granular. Individual grains are visible and feel gritty. Grain size ranges from 2 mm to 64 mm. Can be angular to well-rounded.
- Crystal Form
- Not applicable for the aggregate as a whole. Individual mineral grains within the gravel may exhibit their characteristic crystal forms, but most are anhedral due to transport and abrasion.
- Cleavage
- Not applicable for the aggregate. Individual mineral grains may exhibit cleavage (e.g., feldspar, mica).
- Geological Environment
- Fluvial (riverbeds, floodplains, alluvial fans), glacial (moraines, outwash plains), coastal (beaches, marine terraces), lacustrine (lake shores), and colluvial (talus slopes, scree).
Key Facts
- Hardness: Variable (depends on constituent minerals/rocks, e.g., quartz is 7 on Mohs scale)
- Specific Gravity: Variable (typically 2.5 - 2.8 for common rock types)
- Crystal System: Not applicable (aggregate)
- Color: Highly variable, reflecting source rocks
- Luster: Variable, depending on constituent minerals
- Transparency: Opaque to translucent (for individual grains)
- Fracture: Variable (conchoidal for quartz, irregular for many rock fragments)
- Cleavage: Not applicable for the aggregate; present in some constituent minerals (e.g., feldspar)
- Composition: Mixed rock fragments and mineral grains (e.g., quartz, feldspar, chert, granite, basalt, limestone)
Quick Check
- Color: Variable (mixed colors)
- Luster: Variable (dull to vitreous)
- Streak: Not applicable (aggregate of various minerals/rocks)
Physical Characteristics
- Crystal Habit: Not applicable for the aggregate; individual grains are typically anhedral to subhedral.
- Cleavage Type: Not applicable for the aggregate; individual mineral grains may exhibit perfect, good, or poor cleavage.
- Fracture Type: Variable, depending on the constituent materials (e.g., conchoidal for quartz, irregular for many rock fragments).
- Tenacity: Brittle (for individual grains and rock fragments).
- Luster Type: Variable, ranging from dull, earthy, or resinous to vitreous, depending on the dominant mineralogy.
Formation
Gravel forms through the weathering and erosion of pre-existing rocks, followed by transport and deposition by natural agents such as rivers, glaciers, waves, and wind. The clasts are typically rounded or sub-rounded due to abrasion during transport. Accumulations occur in high-energy environments where finer sediments are winnowed away.
Usage
Gravel is a fundamental construction material, used extensively as aggregate in concrete and asphalt, for road construction (base and sub-base layers), railway ballast, drainage systems, landscaping, and as a filter medium. Specific types of gravel are also used in water purification and as decorative stones.
Age Distribution
Ranges from Precambrian to Holocene, depending on the source rocks and depositional environment.
Where to Find
Riverbeds and Alluvial Fans
Commonly found in active and ancient river channels, floodplains, and alluvial fans where water flow has transported and deposited coarser sediments.
Glacial Deposits
Abundant in areas that have experienced glaciation, such as moraines, eskers, kames, and outwash plains, where glaciers have eroded and deposited vast quantities of rock material.
Coastal Environments
Present on beaches, marine terraces, and coastal plains, where wave action sorts and deposits sediments.
Quarries and Aggregate Pits
Extensively mined from natural deposits in quarries and aggregate pits for construction purposes. These sites are often located near major population centers or transportation routes.
Finding Tips
Look for High-Energy Environments
Gravel typically accumulates in environments where water or ice currents are strong enough to transport larger clasts and winnow away finer sediments. Riverbeds, glacial outwash, and wave-swept beaches are prime locations.
Examine Grain Size and Roundness
Gravel is defined by its grain size (2-64 mm). Observe the degree of rounding; well-rounded clasts suggest significant transport, while angular clasts indicate shorter transport or proximity to the source.
Consider Source Rock Geology
The composition of gravel reflects the geology upstream or upslope from its depositional site. Identifying the types of rocks and minerals present can help trace its origin.
Similar Rocks
Sand
Sand (unconsolidated sediment)
Also known as: Mixed Mineral Grains
Cobble
Cobble (unconsolidated sediment)
Also known as: Large Rock Fragments
Conglomerate
Conglomerate
Also known as: Puddingstone (for some varieties)
Breccia
Breccia
Also known as: Angular Rock Fragments
Scientific Classification
- Mineral Class
- Not a mineral; it is a rock/sediment type.
- Group
- Clastic Sedimentary Material
- Crystal System
- Not applicable (unconsolidated aggregate)
- Chemical Formula
- Variable (mixture of various mineral and rock compositions)
- Composition
- Heterogeneous mixture of rock fragments and mineral grains, predominantly silicates (e.g., quartz, feldspar) but can include carbonates, chert, and other lithologies.
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