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River gravel is an unconsolidated sedimentary deposit composed of a mixture of rock fragments (clasts) that have been transported and rounded by fluvial processes. The composition of these clasts is highly variable, reflecting the geology of the upstream drainage basin. It commonly includes a mix of igneous rocks (e.g., granite, basalt, rhyolite), metamorphic rocks (e.g., gneiss, schist, quartzite), and sedimentary rocks (e.g., sandstone, limestone, chert). The clasts are typically sub-angular to well-rounded, and their size ranges from granules (2-4 mm) to boulders (>256 mm), with pebbles (4-64 mm) and cobbles (64-256 mm) being most common. The matrix, if present, consists of sand, silt, and clay.
How to Identify
- Color
- Highly variable, depending on the constituent rock types. Can be shades of grey, brown, black, white, red, green, or multicolored.
- Luster
- Variable, depending on the constituent minerals and rock types. Can range from dull to vitreous, earthy, or greasy.
- Texture
- Clastic, unconsolidated. Clasts are typically sub-angular to well-rounded, with a smooth to rough surface. Grain size is gravel-sized (greater than 2 mm).
- Crystal Form
- Not applicable to the aggregate as a whole. Individual clasts may exhibit relict crystal forms from their parent rocks (e.g., euhedral crystals in granite clasts).
- Cleavage
- Not applicable to the aggregate. Individual mineral grains within clasts may exhibit cleavage (e.g., feldspar, mica).
- Geological Environment
- Fluvial environments: river beds, point bars, channel lag deposits, floodplains, alluvial fans, and terraces.
Key Facts
- Hardness: Variable, depending on the hardest constituent clasts (e.g., quartz, ~7 on Mohs scale).
- Specific Gravity: Variable, typically 2.6-2.8 g/cm³ for common silicate rocks, but can vary with density of constituent clasts (e.g., denser mafic rocks or heavy minerals).
- Crystal System: Not applicable (aggregate of various rock types).
- Color: Polychromatic, reflecting the diverse lithologies.
- Luster: Variable, from dull to vitreous, depending on clast composition.
- Transparency: Opaque to translucent for most clasts; some quartz clasts may be transparent.
- Fracture: Variable, depending on the constituent clasts (e.g., conchoidal for quartz, irregular for many others).
- Cleavage: Not applicable for the aggregate; individual mineral grains within clasts may exhibit cleavage.
- Composition: A heterogeneous mixture of rock fragments (igneous, metamorphic, sedimentary) and mineral grains (e.g., quartz, feldspar, mica, amphibole, pyroxene, chert, limestone, sandstone, granite, basalt).
Quick Check
- Color: Highly variable (polychromatic)
- Luster: Variable (dull to vitreous)
- Streak: Not applicable for the aggregate; individual clasts will have their own streak.
Physical Characteristics
- Crystal Habit: Not applicable (aggregate). Individual clasts retain habits from their parent rocks.
- Cleavage Type: Not applicable (aggregate).
- Fracture Type: Variable, depending on the constituent clasts.
- Tenacity: Brittle for most individual clasts.
- Luster Type: Variable, from dull to vitreous, earthy, or greasy.
Formation
River gravel forms through the erosional and transportational processes of fluvial systems. Rocks from upstream sources are weathered (physically and chemically), broken into smaller fragments (clasts), and then transported downstream by the river's current. During transport, these clasts undergo abrasion, which rounds their edges and sorts them by size and density. When the river's energy decreases (e.g., in wider, flatter sections, meander bends, or at the mouth of a river), these clasts are deposited, forming gravel bars, floodplains, and channel lag deposits.
Usage
River gravel is extensively used in construction as aggregate for concrete, asphalt, and road base. It is also used for landscaping, drainage systems, filtration media, and as a source of sand and finer sediments. Historically, it has been a source of placer deposits for gold and other heavy minerals.
Age Distribution
Variable, ranging from Holocene (currently forming) to ancient (reworked from older deposits). The age of individual clasts can span billions of years, reflecting the source rocks.
Where to Find
River Beds and Banks
Active river channels, especially in mountainous or hilly regions, and along the banks of mature rivers where energy fluctuations lead to deposition.
Alluvial Terraces
Elevated, flat areas adjacent to river valleys, representing former floodplains or river beds that have been incised by subsequent river downcutting.
Alluvial Fans
Fan-shaped deposits formed where a confined stream emerges from a mountain front onto a flatter plain, causing a rapid decrease in velocity and deposition of coarse sediment.
Floodplains
Flat land areas adjacent to a river, formed by the deposition of sediment during floods. While often dominated by finer sediments, coarser gravel can be found in channel lag deposits or near the main channel.
Finding Tips
Look for Active Fluvial Environments
The most common place to find river gravel is in and around active rivers and streams. Look for gravel bars, especially on the inside bends of meanders where current velocity is lower.
Examine Terraces and Old River Channels
Ancient river systems leave behind deposits. Look for elevated, flat areas adjacent to modern rivers, which may be old river terraces containing well-rounded gravels.
Observe Clast Roundness
River gravel is characterized by its rounded to sub-rounded clasts due to abrasion during transport. This helps distinguish it from angular colluvial deposits or poorly sorted glacial till.
Consider Upstream Geology
The composition of river gravel reflects the geology of the upstream drainage basin. If you know the types of rocks upstream, you can anticipate what types of clasts you might find in the gravel.
Similar Rocks
Glacial Till
Till
Also known as: Boulder Clay
Colluvium
Colluvium
Also known as: Slope Wash
Beach Gravel
Littoral Gravel
Also known as: Coastal Gravel
Scientific Classification
- Mineral Class
- Not a mineral; it is a rock aggregate.
- Group
- Sedimentary Rock (Clastic, Unconsolidated)
- Crystal System
- Not applicable.
- Chemical Formula
- Highly variable, reflecting the diverse mineralogy of the constituent clasts.
- Composition
- Polymictic assemblage of rock fragments and mineral grains, typically dominated by silicates (e.g., SiO2 from quartz, various silicates from feldspars, micas, amphiboles, pyroxenes) and carbonates (if limestone/dolostone clasts are present).
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