Stratigraphy is the study of layers: the principle, long established in geology, that in undisturbed sedimentary sequences, deeper layers are older than shallower layers.

This law of superposition — older below, younger above — is the foundation of all geological and archaeological dating by position.

It was Orr’s reading of the Arlington Springs stratigraphy that told him, before any laboratory test, that the bones came from a very old level.

Eleven meters — 37 feet — is a substantial depth in an alluvial sediment sequence.

It represents the accumulated deposition of thousands of years of stream activity and sediment transport in Arlington Canyon.

The deposits that filled 37 feet above the bone-bearing layer were laid down after the bones were incorporated into the sediment.