In a vertical sequence of beds, which beds are oldest?

• A. Bottom beds
• B. Top beds
• C. Middle beds
• D. Random beds

Answer: (A)

In a vertical stack of sedimentary beds, the oldest beds are at the bottom because sediments accumulate in layers over time, with each new layer forming on top of what already exists. The bottom layer forms first, and as more material settles, newer layers are deposited above it, making the upper beds progressively younger. In an undisturbed sequence, this vertical order directly records the passage of time, so the oldest remains lie at the bottom and the youngest at the top. If the sequence were later disturbed by tilting, erosion, or faulting, this simple pattern could be altered, but in a straightforward vertical arrangement, the bottom beds are the oldest.