Grades 3–5 · Earth & Space
How can sediment help preserve evidence of life?
A shell on a beach may break, dissolve or wash away. Occasionally, sand or mud covers it before it is destroyed. Burial can protect the shell from some scavengers and moving water, giving evidence of life a chance to survive.
More sediment may collect on top. The weight of later layers presses the buried grains together. Minerals carried by water can cement grains, helping turn sediment into rock. A shell trapped inside can become part of the record held in that rock.
Fossils form in several ways. Original hard material may survive, minerals may replace it, or a shell may disappear while leaving its shape as a hollow mold. If minerals or sediment later fill that hollow, they can make a cast of the shape. A footprint can preserve an animal’s action even without its body.
The shell stays at the same buried level while newer layers accumulate above it. A mold preserves shape rather than the original shell.
The drawing follows one possible shell-burial sequence through undisturbed layers. It does not promise that every buried shell becomes a fossil. Heat, erosion and other changes can erase evidence later. A fossil is remarkable because something from a living world survived a long series of chances to be lost.
About this explanation
Starting knowledge: Recognize sediment layers and distinguish a living thing from a trace it leaves.
Sources
- National Park Service — How Fossils Form
- National Park Service — Taphonomy—Death & Decay
- National Park Service — Mold, Casts, and Steinkerns