On Top of a Forest, on the Bottom of a Lake

Two postcards overlapped, the left with a picture of a wayside and a view of the Florissant valley. The left postcard shows lines and a stamp with a wasp on it. — Once a forest, once a lake, now a valley!
NPS

This location is along the bottom of the Florissant valley looking north toward beautiful Crystal Peak. Below the surface and along the sides of the valley are rocks that preserve the ancient Eocene forest and lake. This schematic diagram illustrates the 34 million year old rock layers known as the Florissant Formation. One of these layers was a volcanic lahar that flowed into the valley and quickly buried a forest of redwood trees.

A schematic of the rocks in the Florissant Valley with emphasis on the Florissant formation. The caprock conglomerate is labeled A, the lower and middle shale is labeled B, and the lower mudstone is labeled C. — Schematic cross-section through the Florissant valley.
Geocorps: Emily Thorpe; GIP: Mariah Slovacek

The other layers formed when this valley became blocked by a volcanic dam and the valley flooded. The resulting lake was filled over thousands of years by sediments including volcanic ash, clay, diatoms, and debris flows that accumulated on the bottom to form alternating layers of shale and conglomerate.