Lesson 6 of 6 · Superposition, cross-cutting & incomplete order
Reconstruct a history, including what is missing
10 minute lesson · 3 checks along the way
Your goal for this lessonBuild a justified relative sequence, recognize missing intervals, and preserve uncertainty where two events cannot yet be ordered.
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Start with relationships rather than a story
Geological reconstruction begins by recording contacts and structures. In an undisturbed depositional sequence that has not been overturned, an upper layer was deposited after the layer beneath it. This is . A structure that cuts an older feature formed or moved after that feature existed. These relationships establish ; neither principle supplies a calendar date without additional evidence.
Conditions belong in the rule. An intrusive sheet can be younger than rocks both above and below it, and overturned layers reverse their present vertical order. A buried surface can record or a pause in , called an unconformity. Missing layers do not mean nothing happened. They may mark an interval during which earlier deposits were removed, leaving only a boundary as its surviving trace.
Check 1 of 3
10 pointsUse the reading above. You can try again without losing points.
Read a crater wall on Pluto with restraint
New Horizons photographed layered crater walls on Pluto in 2015. The image release identified visible layering while leaving open whether it extended locally, regionally, or more broadly. An impact can expose pre-existing material and redistribute it in . A layer in a crater wall is therefore an opportunity to investigate older material, but brightness differences alone do not identify every composition or emplacement process.
The same release discussed dark ejecta around a comparatively fresh-looking crater and uncertain origins for nearby linear features. A useful field notebook would separate those observations from interpretations. Write “a contact is traceable across this wall” before “a global ocean deposited this bed.” Regional imaging, topographic information, and spectral measurements could test continuity and composition; one detailed frame cannot establish a complete planet-wide timeline.
New Horizons’ July 14, 2015 LORRI observations show layered crater walls on Pluto. The released image adds a six-mile scale bar; its top points northwest. The source distinguished visible layering from unsettled questions about its extent and the origin of nearby linear features.
Look closely
Choose a traceable band inside a crater wall. Which larger claim would require observations beyond this frame?
See what to notice
A claim that the same layer extends across all of Pluto needs regional continuity and compositional evidence. This frame can reveal local layering; a global depositional history or a unique origin does not follow from that visibility alone.
Work through a decision
Assemble only the order that the contacts require
Consider our invented cross section. Sedimentary layer A lies below B in an upright sequence. D cuts both. An erosional surface E truncates the top of D and B, and an undisturbed cap C rests on E. These contacts require A, then B, then D, then erosion E, then deposition C. E is an event represented by a surface, not another thick rock layer.
A F later offsets C on the left, and crater K excavates C on the right. They do not intersect, and neither deposit links them. Both postdate C, but their mutual order is unresolved: F could precede K, K could precede F, or their histories could overlap. Preserve that uncertainty as two branches. Do not force a single seven-item sequence merely because a worksheet provides seven labels.
A history can branch when evidence does
An original hypothetical geological section and order diagram, not a mapped world or a calibrated time axis. Shapes are schematic. Each arrow means earlier than; the separate branches preserve the missing relationship between fault movement F and crater K.
Open full-size diagram in a new tab: A history can branch when evidence doesCheck 2 of 3
10 pointsUse the reading above. You can try again without losing points.
Add dates without replacing the geological argument
Suppose reliable dates in a separate idealized example place a truncated unit at 80 million years ago and its overlying cap at 50 million years ago. The intervening erosion occurred after the lava and before the cap, within that bracket. The evidence does not make its age exactly 65 million years. Dating uncertainties would widen or complicate the bracket, and an inherited grain could be older than its host deposit.
Finish by listing the observation that supports each arrow in your sequence and the observation needed for each unresolved branch. Finding ejecta from K displaced by F could establish later fault movement; finding an unbroken K deposit across F could constrain local activity differently. This habit turns an attractive geological story into a revisable reconstruction. A good history includes justified events, missing intervals, and clearly identified questions for the next measurement.
Check 3 of 3
10 pointsUse the reading above. You can try again without losing points.
Put it together
Your lesson progress
Each check connects to a part of the story. Complete all 3 to earn your 25-point lesson bonus.
Go to the source
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