How to identify River Gravel

To correctly identify River Gravel (Mixed Sedimentary and Igneous Clasts), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Highly variable, depending on the constituent rock types. Can be shades of grey, brown, black, white, red, green, or multicolored.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Variable, depending on the constituent minerals and rock types. Can range from dull to vitreous, earthy, or greasy.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the aggregate. Individual mineral grains within clasts may exhibit cleavage (e.g., feldspar, mica).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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).

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.

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