How to identify Weathered Sedimentary Rock

To correctly identify Weathered Sedimentary Rock (Sedimentary Rock (likely sandstone or conglomerate with significant weathering)), 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, often duller than fresh rock. Can be reddish-brown (due to iron oxidation), yellowish, grey, or bleached white. Mottling is common.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, reflecting the altered and often porous nature of the rock. Original mineral luster may be obscured.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Friable, crumbly, granular, or earthy. Grains may be easily dislodged. Original clastic texture (sand grains, pebbles) is usually still discernible but less coherent. Pitting and solution features may be present.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for a rock; individual mineral grains within the rock may show their original crystal forms, but these are often corroded or obscured by weathering products.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable for a rock. The rock itself does not exhibit cleavage, but individual mineral grains (e.g., feldspar) within it might. Weathering often reduces the coherence along original bedding planes or fractures.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Exposed outcrops, road cuts, river banks, coastal cliffs, arid and humid environments, areas of tectonic uplift and erosion. Found wherever sedimentary rocks are subjected to surface weathering processes.

Key Facts

Hardness

Highly variable, generally much lower than fresh rock (e.g., 1-5 on Mohs scale, depending on degree of weathering and original cement). Can be easily scratched with a knife or even fingernail if severely weathered.

Specific Gravity

Variable, generally lower than fresh rock due to increased porosity and alteration of denser minerals. Typically 1.8-2.5 g/cm3.

Crystal System

Not applicable for a rock; refers to individual mineral components.

Color

Variable, often muted or stained (e.g., reddish-brown, yellowish, grey, white).

Luster

Dull, earthy, or powdery.

Transparency

Opaque to translucent in thin sections; individual grains may be transparent.

Fracture

Irregular, earthy, or granular. May follow original grain boundaries.

Cleavage

Not applicable for a rock; individual mineral grains may exhibit cleavage.

Composition

Primarily quartz, feldspar, rock fragments, and clay minerals (kaolinite, illite, smectite) as weathering products. Iron oxides/hydroxides (goethite, hematite) are common staining agents. Original cements (calcite, silica, iron oxides) may be partially dissolved or altered.

Physical characteristics

• Crystal Habit: Not applicable for a rock; refers to individual mineral components.
• Cleavage Type: Not applicable for a rock.
• Fracture Type: Irregular, granular, earthy.
• Tenacity: Friable, brittle, crumbly.
• Luster Type: Dull, earthy.

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