xkcd.WTF!?

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 standing 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 being solidified 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. 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 has added a postscript (whether on the same stone or a separate one is unclear) suggesting that the aliens/future-earth-based-lifeforms may have access to time travel technology, and inviting them to her birthday party (next) Saturday. The invitation does not seem to give an indication when it was written, and therefore which Saturday is meant, so, even if the finders were able to time travel, it might still be non-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 which 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.