Gaps in the Rock Record

Demo 15 of 17 · Part four · Geology · Stage 1

A continuous section. Does absence mean extinction?

Does a species' last appearance in sampled rock mark where it went extinct?

Before you start. Two measurements stay separate throughout this activity: rock thickness in metres records surviving strata, and model time in Myr (one Myr is one million years) is the elapsed clock this authored model runs on. A fossil specimen marks where and when it was buried — not a full record of the organism's whole existence.

Predict

Guess before you sample: does finding where a fossil stops appearing settle whether the species went extinct there?

Try

Press sample on every surviving layer in Section A, then Show supplied age evidence to see the section's Myr chronology.

Takeaway

A local last observation in sampled rock is where the record stops, not a proven extinction moment.

Check yourself

Asteria is sampled in the lower two units of Section A but not in Gold sandstone above them. Does that alone establish that Asteria went globally extinct partway up the section?

Demo 15 of 17 · Part four · Geology · Stage 2

A missing interval, and where its fossils went.

Is a rock gap always someone's last moment, and do a gap's missing fossils end up sitting at its boundary?

Before you start. A contact is the surface between two rock units. Where rock — and therefore time — is missing across it, that missing interval is a hiatus. Section B's hiatus is produced by an authored erosion event, which deletes rock (and any fossils inside it) rather than moving it anywhere. Section A handed forward that a local record has an edge; here the edge is a gap rather than the top of the section.

Predict

Guess before you look: once you find rock missing here, will the surviving evidence also tell you why it is missing?

Try

Press Show supplied age evidence to reveal the gap, then sample Red siltstone and Cross-bedded sand on either side of it.

Takeaway

Erosion deletes rock and its fossils outright; nothing is relocated to the boundary that erosion leaves behind.

Check yourself

Erosion removed the top of Red siltstone and all of Pale limestone in Section B. Are the fossils that were once in that removed rock now sitting at today's contact, waiting to be found there?

Demo 15 of 17 · Part four · Geology · Stage 3

Two sections, one gap between them.

Does a second section that keeps depositing through a first section's gap settle what the gap means?

Before you start. Sections C and D start identically, then diverge: C has a pause (no deposition, but no erosion either — a hiatus with a different cause than Section B's), while D keeps accumulating rock through that same span. Stage 2 showed one cause of a gap; this stage's pause shows a second, equally valid one.

Predict

Guess before you compare: if Section D preserves fossils from exactly the span missing at C, does that settle the question for the species everywhere?

Try

Show supplied age evidence for both sections, then sample below C's gap and the matching Green mudstone layer in D.

Takeaway

A second section can show what a gap hides locally, but two local sections still cannot certify a global claim.

Check yourself

Section D preserves fossils from exactly the span missing at Section C. Do these two local sections, compared, prove a global claim about where the species lived?

Stage	What happens	Time
1 — Section A	A continuous three-layer record. Sample every layer, reveal supplied ages, and test whether a fossil's last appearance settles extinction.	~3 min
2 — Section B	An erosional gap. Reveal the missing interval, sample above and below it, and test whether the gap explains its own cause.	~3 min
3 — Sections C & D	A paired comparison: C pauses, D keeps depositing. Sample both to test what a second section can and cannot settle.	~3–4 min