What does arkose mean?
Arkose is a coarse-grained sedimentary rock — specifically a sandstone distinguished by its high content of feldspar, which makes up at least a quarter of the grains alongside quartz. It forms when granitic rocks are eroded rapidly and deposited nearby, before the feldspar has time to chemically weather into clay. Because of this, arkose is often described as an immature sandstone, one that records a short journey from mountain to basin. It is typically pink, red, or grey, coloured by the potassium feldspar it contains, and is commonly found in continental deposits such as alluvial fans and river sediments. The word belongs firmly to the vocabulary of geology: students encounter it when learning to classify sandstones, and field geologists use it to interpret ancient landscapes. Its adjective form, 'arkosic', extends the term to any rock or sediment rich in feldspar. For anyone interested in reading Earth's history from stone, arkose is a small but telling clue about how fast mountains crumble.
nounA coarse-grained sandstone containing at least 25% feldspar, derived from the rapid erosion of granitic rocks and typically deposited close to its source.
- A coarse-grained sedimentary rock (a sandstone) composed largely of quartz and abundant feldspar grains, formed by the rapid weathering and erosion of granitic source rocks.
"The cliff face exposed a thick band of pink arkose, its feldspar grains glinting in the afternoon light."
"The river had cut through layers of arkose, revealing feldspar crystals that sparkled like sugar."
"Geologists identified the deposit as arkose because of its unusually high feldspar content."
The plural is rare and appears mainly in geological literature when comparing multiple arkose formations or samples.
"The region contains several distinct arkoses of differing ages."
Arkose is essentially a sandstone that never got the chance to fully decompose — it's granite caught mid-transformation, with up to a quarter of it still raw feldspar.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of arkose
The word 'arkose' derives from the French 'arkose', coined in the early nineteenth century from the Greek 'arkhos' meaning 'first' or 'chief', a reference to the rock's granitic, primary character. It entered English geological usage through French scientific literature as sedimentary petrography developed as a discipline. The derived adjective 'arkosic' shares the same root.
Related word forms
How arkose is actually used
A technical term used almost exclusively in geology and earth science; rarely encountered outside academic or field contexts. The related adjective 'arkosic' describes rocks or sediments with arkose-like composition.
Easily confused with arkose
Greywacke is a hard, dark sandstone rich in rock fragments and matrix, whereas arkose is a pale sandstone defined by its high feldspar content.