How to identify Weathered Sedimentary Rock with Lichen

To correctly identify Weathered Sedimentary Rock with Lichen (Sedimentary rock (likely sandstone or siltstone) with lichen 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

   The rock matrix color can vary widely (e.g., tan, brown, red, grey, white) depending on its mineralogy and cement. Weathering often leads to duller, more muted tones, or iron-staining (reds, yellows). Lichens add patches of green, grey, yellow, orange, or black, often forming distinct patterns or crusts on the surface.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy for the rock matrix. Lichens typically have a matte or slightly waxy appearance.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The underlying rock will have a clastic texture, ranging from fine-grained (siltstone, grain size 0.0039-0.0625 mm) to medium-grained (sandstone, grain size 0.0625-2 mm). Weathering can make the surface friable, granular, or pitted. Lichens present as crusts, leafy structures, or branching forms on the surface, obscuring the rock's original texture in places.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral grains within the sedimentary rock are typically anhedral to subhedral, often rounded or sub-rounded due to transport. Lichens do not exhibit crystal forms but grow in thalli (body forms) characteristic of their species.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   The sedimentary rock itself typically lacks true cleavage, though it may exhibit parting along bedding planes. Lichens do not have cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in terrestrial environments where sedimentary rocks are exposed to the atmosphere, moisture, and sunlight. This includes outcrops, cliffs, boulders, arid and semi-arid regions, temperate forests, and polar environments. Lichens are pioneer organisms and can thrive in harsh conditions.

Key Facts

Hardness

Variable. The underlying sedimentary rock can range from soft (e.g., friable siltstone, Mohs 1-2) to moderately hard (e.g., well-cemented quartz sandstone, Mohs 6-7). Weathering generally reduces surface hardness. Lichens themselves are soft and easily scraped.

Specific Gravity

Variable, typically 2.2-2.8 g/cm³ for sandstone/siltstone, depending on mineralogy and porosity. Lichen biomass has a very low density.

Crystal System

Not applicable for the rock as a whole or for lichens. Individual mineral grains within the sedimentary rock may exhibit their own crystal systems (e.g., quartz is trigonal).

Color

Rock matrix: variable (tan, brown, red, grey). Lichens: green, grey, yellow, orange, black.

Luster

Rock matrix: dull to earthy. Lichens: matte to slightly waxy.

Transparency

Opaque for the rock and lichens.

Fracture

Conchoidal to irregular for individual grains; granular or irregular for the rock mass. Lichens do not fracture in a geological sense.

Cleavage

Absent in the rock mass, though individual mineral grains may have cleavage. Lichens do not have cleavage.

Composition

Sedimentary rock: primarily quartz, feldspar, clay minerals, rock fragments, and various cementing agents (silica, calcite, iron oxides). Lichens: fungal hyphae, algal/cyanobacterial cells, and associated organic compounds.

Physical characteristics

• Crystal Habit: Not applicable for the rock as a whole or for lichens. Detrital grains are typically clastic (rounded to angular).
• Cleavage Type: None for the rock mass; individual mineral grains may exhibit cleavage (e.g., feldspar).
• Fracture Type: Granular, irregular, or conchoidal (for quartz grains).
• Tenacity: Brittle for the rock, especially when weathered. Lichens are generally flexible when moist, brittle when dry.
• Luster Type: Dull to earthy.

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