onlap
Rare (Specialist/Technical)Formal, Scientific
Definition
Meaning
A geological term describing the relationship between sedimentary rock layers where younger layers progressively extend over the edge of an older, underlying layer.
More broadly, a stratigraphic configuration where sedimentary units are deposited in a series of layers that progressively expand over the topographic highs of an older, underlying surface.
Linguistics
Semantic Notes
"Onlap" is a noun describing a specific geometric relationship in stratigraphy. It is often contrasted with "downlap" and "overlap." It describes the termination pattern of strata against an older surface.
Dialectal Variation
British vs American Usage
Differences
No significant differences in meaning or spelling. Both varieties use the term exclusively in geological/stratigraphic contexts.
Connotations
Purely technical, neutral connotations in both varieties.
Frequency
Extremely low frequency in both; used almost exclusively by geologists and earth scientists.
Vocabulary
Collocations
Grammar
Valency Patterns
The [sedimentary unit] shows onlap onto the [underlying surface].A well-defined onlap is observed in the [formation name].Vocabulary
Synonyms
Neutral
Weak
Vocabulary
Antonyms
Usage
Context Usage
Business
Not used.
Academic
Exclusively used in geology, sedimentology, and stratigraphy papers and textbooks to describe rock layer geometries.
Everyday
Never used in everyday conversation.
Technical
Core technical term in earth sciences for describing the termination of strata against an underlying surface.
Examples
By Part of Speech
verb
British English
- The transgressive sediments are seen to onlap the underlying limestone platform.
American English
- The sandstone units progressively onlap the eroded paleo-high.
Examples
By CEFR Level
- The seismic data clearly shows an onlap termination pattern at the base of the sequence.
- Marine sediments often onlap onto continental shelves during periods of rising sea levels.
Learning
Memory Aids
Mnemonic
Imagine new blankets (layers) being laid ON and LAPPING over the edge of an old, lumpy mattress (the older rock surface).
Conceptual Metaphor
LAYERS AS BLANKETS (new blankets spread over the edge of an old mattress).
Watch out
Common Pitfalls
Translation Traps (for Russian speakers)
- Do not confuse with general terms for "overlap" like "перекрытие" or "нахлест". "Onlap" is a specific stratigraphic concept. A closer translation might be "стратиграфическое перекрытие" or the borrowed term "онлап" in technical contexts.
Common Mistakes
- Using it as a verb (e.g., 'The sediments onlap the surface' – while understood in geology, the primary dictionary entry is as a noun).
- Confusing it with the more general word 'overlap'.
- Misspelling as 'on-lap' or 'on lap'.
Practice
Quiz
What does 'onlap' specifically describe in geology?
FAQ
Frequently Asked Questions
No, it is a highly specialized term used almost exclusively in geology and earth sciences.
In technical geological writing, it is sometimes used as a verb (e.g., 'the unit onlaps the basement'). However, its primary dictionary entry is as a noun.
The primary opposite in stratigraphy is 'downlap', where strata terminate by descending and pinching out against a surface.
Yes, significantly. 'Overlap' is a general term meaning to extend over and cover part of something. 'Onlap' is a specific stratigraphic term describing the termination geometry of sedimentary layers against an older, often uneven surface.