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| Deposition |
 Title text: P.S. If you have time travel, come to my birthday party Saturday! |
Explanation
A continental margin is the place on the edge of a continent where the continental crust is underwater, covered by relatively shallow coastal waters. The scene is ambiguous, but it is feasible that Ponytail is stood on the back-shore littoral zone next to the rivermouth, such that she is actually a stone's throw away from the sea. As such, the stones she throws off to the right end up embedded directly into the soft shoreline/sediments without having to risk further erosion from all the other stones and gravel also sitting in the riverbed or being tumbled down it.
The continental margin is said to be passive, which means that it is not currently undergoing subduction, where the oceanic crust slips under the continental crust, or a strike-slip fault, where one slides along the other, both of which can mechanically or thermally transform any seafloor material. Absent such occurrences, this causes piles of sediment to accumulate on the continental shelf with a minimum of additional geological disturbance.
This rock's eventual resting place in the sediment seems destined to be compressed by further overlying sedimentation and solidify over geologic timescales into shale or other similar sedimentary rock types, presuming that the future movement of further sediment and relative local changes of sea-level and shoreline keep adding more material. As shown, 100 million years later the sea level has gone down (and/or the bedrock has risen), re-exposing the strata, and recent erosion/quarrying has caused it to become a cliff face that eventually re-exposes the original rock that Ponytail threw into the river, apparently just at the right time and place to be discovered or uncovered by aliens/far-future-earthlings.
These beings appear to be digging with relatively primitive hand-tools that are strangely anthropocentric and relatively inefficient, given the apparent use of antigravity personal conveyors with mechanical manipulators, and have found a rock. Whether or not they fully comprehend it, this seems to be one of those left with a still visible carved message by Ponytail, saying "This bedrock inspected by No. 5". This is a parody of a typical quality control label left attached to (or hidden within) clothing, to reassure any purchaser and/or help identify which manufacturing and inspection path any newly discovered product defect had passed through.
In the title text, Ponytail suggests that the aliens/future-earth-based-lifeforms may have access to time travel technology, and invites them to her birthday party (next) Saturday, which suggests that the "inspected by" message is just one of several different speculative statements engraved on any particular stone. The invitational one does not seem to give an indication which Saturday is meant so, even if the finders would be able to time travel, it might still not be trivial for them to attend the birthday party. This also presupposes that the future discoverers understand the concept of the seven-day week cycle and have no trouble reading the invite, both remaining legible and not requiring impractical levels of translation from ancient "English". It may also be referencing the time travel party held by Stephen Hawking, in which he held a party where he hoped time travelers would attend. Hawking released invitations to the party only the following day, so only a time traveler would be able to attend the party.
Transcript
- [Cueball approaches Ponytail, chiselling a rock on a shoreline next to a river with shallow rolling hills in the background]
- Cueball: What are you doing?
- Ponytail: This river empties onto a passive continental margin.
- [Cueball and Ponytail stand talking, Ponytail holding several flat rocks, in an otherwise empty and frameless panel]
- Ponytail: If I chisel notes onto these rocks and throw them into the sea, they might be incorporated into some shale cliff in the distant future.
- [Silhouetted scene of Ponytail as she throws multiple rocks off frame to the right, Cueball watching from behind her]
- [From off-panel, sound effect of a rock hitting water:] PLOP
- [Two 'bug-eyed aliens', sitting in personal 'hover-saucers' look rightwards at an exposed rock-face. A pick and shovel are left stuck in the ground, and one of the 'saucers' sports a mechanical arm currently holding a loose fragment of rock. There are three question marks above the alien on the left and two question marks next to the alien on the right]
- [Panel label:] 100 million years later...
- [Text originating from the held rock fragment:] This bedrock inspected by No. 5
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