Where is Weathered Sedimentary Rock found?
Sedimentary rock (likely sandstone or shale, heavily weathered)
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Open the appWeathered Sedimentary Rock occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Weathered Sedimentary Rock
Global
Weathered sedimentary rocks are ubiquitous globally, found wherever sedimentary rock formations are exposed to the Earth's surface and subjected to weathering processes. This includes virtually all continents and diverse climatic zones.
Arid and Semi-Arid Regions
Physical weathering (e.g., thermal expansion/contraction, salt wedging) is prominent, leading to granular disintegration of sandstones and shales.
Humid and Tropical Regions
Chemical weathering (e.g., hydrolysis, dissolution, oxidation) is dominant, leading to extensive alteration of minerals and formation of clay-rich saprolites from various sedimentary protoliths.
Temperate Regions
Both physical (e.g., freeze-thaw) and chemical weathering processes are active, leading to a range of weathered sedimentary rock types.
Collecting tips
- Look for Discoloration and Softness. Weathered rocks often exhibit altered colors (reds, yellows, browns) and are noticeably softer or more crumbly than their fresh counterparts. Use a rock hammer or even your hand to test friability.
- Observe Outcrop Features. Exposed rock faces will show signs of erosion, such as rounded edges, pitting, exfoliation, or the presence of loose debris at the base of the outcrop. Look for areas where the rock appears to be breaking down into smaller fragments or soil.
- Check for Secondary Mineral Growth. Weathering can lead to the formation of new minerals, such as iron oxides (limonite, goethite), clay minerals, or gypsum, which may appear as coatings, stains, or soft masses within the rock.
- Consider the Geological Context. Weathering is most pronounced at the surface. Look for weathered zones at the top of rock units, along fractures, or where water has had prolonged access.
Quick Check
- Color:
- Variable (reddish-brown, yellowish, grey, mottled), often duller than fresh rock.
- Luster:
- Dull, earthy, or absent.
- Streak:
- Variable, often light brown, yellowish, or the color of the dominant clay/oxide minerals.
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