Discriminate a faulted site from an erosional channel using independent cores

1. Define the evidence and process

A site reconstruction combines observations that constrain different parts of a history. Surface relief and variable gravel thickness can be compatible with several mechanisms. A correlated subsurface marker provides a different test: it can reveal whether displacement extends below a shallow erosional surface.

2. Connect mechanism, geometry and boundary

The original invented section contains a planar ash marker beneath younger gravel. Candidate F is a post-gravel fault that offsets both marker and gravel. Candidate C is a channel eroded within the gravel above an intact marker. Paired boreholes at a common datum report the same positively identified marker at west 80 m and east either 70 m, 80 m, or unrecovered. The drawings and observations are supplied teaching records, not actual drilled cores.

Within these deliberately restricted candidates, a 10 m marker offset supports F while equal elevations support C. A missing second core leaves both candidates possible: nonrecovery is not an elevation measurement and cannot be entered as zero. Gravel-thickness variation alone was insufficient, which is why the independent deeper marker was requested.

Real geological interpretation would also test whether the two markers truly correlate, whether original depositional relief or differential compaction existed, and whether folding or more complex faults could produce the pattern. A supported history does not by itself prove current fault activity or a safe foundation. Retain alternatives and identify which new observation could overturn the interpretation.

3. Reconstruct the worked packet

For west marker 80 m and east marker 70 m, the observed 10 m offset is compatible with F and incompatible with the stipulated intact planar marker in C. This conclusion uses both the new measurement and the candidate assumptions; it does not establish an actual active fault.

4. Predict and investigate

Predict first: Does an unrecovered marker provide the same evidence as an observed marker at the expected elevation?

Compare all three core packets against both histories. Retain an observation-versus-assumption table and candidate exclusions. Propose a correlation or geometry check that could distinguish model failure from a new geological mechanism.

5. Test changed evidence

Apply the evidence to a changed case

If the east marker is not recovered, how many candidates remain and what should be recorded for its elevation?

Retain the evidence frame, calculations and assumptions before opening public feedback.

Compare a worked response

Both candidates remain. Record elevation as unknown, not 0 m and not 80 m. A deeper or differently located supplied core, or an independent correlation test, may discriminate them; a missing value cannot close the history.

6. Retain a bounded decision

Preserve the first plan, source identity, original observations, units and competing interpretations. All numerical packets and diagrams in this category are invented teaching cases, not measured site conditions. Retain earlier actual USGS archives separately; do not merge unrelated sites into an invented actual history. Human review, unfamiliar transfer and any chosen supervised practical pathway remain separate. No mining, treatment, slope access or environmental clearance is authorized by these exercises.

Mechanism reference, checked 30 October 2026. Original prose, calculations, questions and diagrams.