How to identify Weathered Sedimentary Rock (with lichen/algae growth)

To correctly identify Weathered Sedimentary Rock (with lichen/algae growth) (Sedimentary Rock (likely sandstone or siltstone) with lichen/algae growth), 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. The underlying rock color (tans, browns, reds, grays) will be present, but overlaid by patches of green (algae, some lichens), gray, yellow, orange, black (various lichens).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, often obscured by biological growth. The rock itself may have a dull luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The rock surface can be rough, granular, friable, or powdery due to weathering. The biological growth adds a textured, often crusty or slightly raised feel, or a smooth, slimy feel if algae are dominant and wet.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for the rock as a whole; individual mineral grains within the sedimentary rock may show crystal forms, but these are typically clastic (broken) and not well-formed. Lichens and algae do not form crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable for the rock as a whole. Individual mineral grains (e.g., feldspar) may exhibit cleavage, but the rock itself does not have a characteristic cleavage plane. The biological growth has no cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Exposed rock outcrops, cliffs, boulders, and rock surfaces in terrestrial environments, particularly in humid to semi-arid climates where moisture is available for biological growth. Common in areas with moderate to high rainfall, or where fog/dew provides moisture.

Key Facts

Hardness

Variable, depending on the underlying rock and degree of weathering. Fresh quartz sandstone grains are 7 on Mohs, but the weathered rock surface can be very soft (1-3) and friable. Biological growth has no measurable hardness.

Specific Gravity

Variable, depending on the underlying rock. Typically 2.2-2.7 for sandstone/siltstone. Biological growth has negligible impact on bulk specific gravity.

Crystal System

Not applicable for the rock as a whole or the biological growth. Individual mineral grains within the sedimentary rock may be crystalline (e.g., quartz is trigonal).

Color

Mottled, with natural rock colors (tans, browns, grays) overlaid by biological colors (greens, grays, yellows, oranges, blacks).

Luster

Dull to earthy.

Transparency

Opaque.

Fracture

Irregular to conchoidal (for individual quartz grains), but often granular or earthy for the weathered rock mass.

Cleavage

None for the rock mass; individual mineral grains may exhibit cleavage (e.g., feldspar).

Composition

Primarily silicate minerals (quartz, feldspar, clay minerals) for the sedimentary rock, with organic compounds from lichens and algae on the surface. Cementing agents (silica, calcite, iron oxides) are also present in the rock.

Physical characteristics

• Crystal Habit: Not applicable for the rock or biological growth. Clastic grains within the rock are typically anhedral to subhedral.
• Cleavage Type: None for the rock mass.
• Fracture Type: Granular, earthy, or irregular.
• Tenacity: Brittle to friable, depending on the degree of weathering and cementation.
• Luster Type: Dull to earthy.

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