The Riverbed That Only Appends

Sediment has one rule: it never rewrites what’s underneath. A flood drops a centimetre of silt, a season drops another, and the layer below is sealed, untouched, for a few hundred million years. Nothing edits the Devonian. You can only add to the top of the pile, and to read the past you dig downward through everything that came after.

That’s an append-only log, described by a riverbed instead of a database.

In Erlang, where values can’t be mutated at all, the match is almost too neat. Prepending to a list — cons — is the whole operation. Each deposit builds a new bed; the old one still exists, unchanged, because in Erlang everything does.

deposit(Bed, Layer) -> [Layer | Bed].

dig([]) -> ok;
dig([Top | Older]) ->
    io:format("~s~n", [Top]),
    dig(Older).

main(_) ->
    Bed = deposit(deposit(deposit([],
              "Devonian shale"), "Carboniferous coal"), "Permian sandstone"),
    dig(Bed).

Permian sandstone
Carboniferous coal
Devonian shale

The dig reads newest-first, top-down, exactly how you’d core it.

JavaScript will happily let you mutate an array, so append-only becomes a discipline you impose rather than one the language guarantees. Object.freeze turns each bed into rock: every deposit copies the record forward and locks it.

const deposit = (bed, layer) => Object.freeze([...bed, layer]);

let bed = [];
for (const layer of ["Devonian shale", "Carboniferous coal", "Permian sandstone"])
  bed = deposit(bed, layer);

for (let i = bed.length - 1; i >= 0; i--)
  console.log(bed[i]);

The cost is the same in both: you keep everything. An immutable log is the whole rock column, not just the top layer — space traded for a history you can never corrupt. Databases pay this bill for audit trails; sediment pays it by being heavy.

Here’s the part that snagged me at the bench today, an air scribe buzzing against an ammonite nodule that had sat under the lamp for nine days. The rock is append-only. Preparation is not. Every pass of the stylus removes matrix that will never come back — a destructive edit on a structure that spent 200 million years being strictly immutable. Which is why you photograph the block before you touch it. The log stops being append-only the moment I switch the tool on, so I keep my own, on the memory card, one frame per layer removed.