Every layer of sediment records where the shoreline was the moment it was deposited. The space available for sediment to fill — the accommodation — is created by two independent processes:
A(t) = dE/dt + S
E(t) = eustatic sea level = Amp · sin(2π t / Period)
S = tectonic subsidence rate (basin floor sinking)
Each timestep the basin receives a fixed sediment volume (the supply-rate control). It is stacked from the river mouth outward: any column still below relative sea level is filled toward sea level first, and only the leftover volume reaches further into the basin — this is why the cross-section progrades as a set of inclined layers (clinoforms) rather than a flat pile.
The balance between how fast sediment fills a column and how fast accommodation opens up decides whether the shoreline advances (regression) or drowns (transgression), which is exactly how real sequence-stratigraphic systems tracts are defined:
- FSST (falling-stage) — sea level itself is falling; the shoreline is forced basinward.
- LST (lowstand) — sea level near its cycle minimum, just turning to rise; sediment still outruns the slow accommodation gain and the shoreline keeps advancing.
- TST (transgressive) — sea level rising faster than sediment can fill the new space; the shoreline retreats landward and marine facies onlap the older coastal-plain deposits.
- HST (highstand) — sea-level rise has slowed near the cycle peak; supply catches up again and the shoreline advances (progrades) over the drowned shelf.
Walther's Law is visible directly in the cross-section: because deposition is continuous and the shoreline sweeps back and forth, the vertical stack of facies you see in any one column is the same sequence you'd cross walking laterally along the shoreline at a single moment — no gaps, only conformable transitions, provided the environments were laterally adjacent when each layer formed. The Wheeler diagram below makes this rigorous: it plots the same basin in time-vs-position space, so a hiatus (a column receiving no sediment that step, drawn as a gap) is visible directly instead of being hidden inside the rock's thickness.
Note: the accommodation-rate readout here is dE/dt + S exactly as defined above — the source 3D model computes that same sum internally but only ever displays the eustatic term, silently dropping the subsidence contribution from the on-screen number even though its own theory box states the two-term formula. Fixed here so the number matches the stated equation.