The backstory
Sixty-five million years, one cell at a time
Petrified wood is not wood that hardened. It is a tree that stood still long enough for quartz to move in and take its place, room by room, without knocking the building down.
Roughly 68 million years ago
A forest, and then a flood
The Rocky Mountains were pushing up to the west, and everything shed off them ran east into a broad, swampy basin — the ground that is now Douglas County. It was warm and wet, closer to the Gulf Coast than to the Colorado you know. Conifers, palms and broadleaf trees grew in it.
Rivers on that plain changed course constantly. A flood would undercut a bank, drop a tree into the channel, and bury it under a foot of sand in a single season. That speed is the whole story: a log that lies on the forest floor is eaten by fungi and insects within a few decades. A log sealed under wet sand is not.
Within years of burial
Sealed away from oxygen
Under saturated sediment there is almost no free oxygen, and the organisms that break wood down cannot work without it. Decay slows to a crawl and then effectively stops. The trunk keeps its shape, its bark, its growth rings, even the tunnels beetles bored through it while it was alive.
The Front Range was volcanically active at the time, and the rivers carried ash along with everything else. That ash is the hidden ingredient. As it weathers underground it releases silica — dissolved quartz — into the groundwater moving through the sediment.
Over thousands to millions of years
Silica moves in, cell by cell
Silica-rich water percolates through the buried wood and, where conditions shift slightly — a change in temperature, acidity, or concentration — the silica comes out of solution and crystallizes inside the wood. It fills the hollow cell lumens first, then infiltrates the cell walls themselves.
This is permineralization, and it is more faithful than the word "replacement" suggests. The mineral is not carving a copy from the outside; it is setting up inside a structure that is still standing, using the wood as a mold. That is why thin sections of good material still show individual cell walls, ray structures and annual rings under a microscope.
The first silica to arrive is usually opal — soft, watery, disordered. Given enough time and pressure it reorganizes into chalcedony and finally into microcrystalline quartz, which is what gives finished petrified wood its hardness of about 7 on the Mohs scale, the same as the quartz in beach sand.
The last few million years
Uplift, erosion, and a stone at your feet
The whole basin was later lifted and tilted, and rain and wind have been stripping it back down ever since. The soft sandstone around the wood erodes far faster than the wood itself, which by now is solid quartz.
So the pieces work their way out of the hillside and are left lying on the surface, in washes and along cut banks, waiting to be picked up. That is where every stone on this site comes from — walked up, one at a time, on the ground around Parker.
Detail work
What survives the process
Growth rings
Countable, and readable. Wide rings mean a good wet year; a tight run of narrow ones means drought. You are looking at weather reports from the Cretaceous.
Cell structure
In well-preserved material the individual tracheids and vessel walls survive at micrometer scale. Specialists can often identify the wood to family from a thin section.
Bark and knots
Where the outer surface was protected, the bark texture comes through. Knots show up as swirls in the grain, exactly as they would in a sawn board.
Insect damage
Beetle galleries and borings are sometimes preserved and infilled with a different generation of silica, so they stand out in a contrasting color.
Later fractures
After the wood had petrified, ground movement cracked some pieces. Those cracks refilled with clean chalcedony — the bright veins cutting across the grain.
What does not survive
The organic material itself, almost entirely. What you are holding is quartz in the shape of a tree, not a tree that got hard.
Chemistry, visible
Where the color comes from
The silica itself is colorless. Every color in a finished piece was donated by trace metals dissolved in the groundwater at the moment the quartz crystallized — which is why two logs from the same ancient forest, buried a few hundred yards apart, can come out looking nothing alike.
It also means color is a local signature. After a few seasons of collecting you can pick up a piece and make a decent guess at which drainage it eroded out of.
- Red and rustIron oxides — hematite, and goethite where the iron was hydrated.
- Honey and ochreIron again, in lower concentration or a more oxidized state.
- Black and greyManganese oxides, or carbon left over from the original wood.
- GreenTrace chromium, nickel or reduced iron — uncommon here, prized when it turns up.
- White and clearNearly pure silica, with no metals along for the ride.
- Pink and mauveIron with a little manganese, or very fine hematite dispersed through the quartz.
Local geology
Why there is so much of it around Parker
The rock unit under most of this area is the Dawson Arkose — a thick pile of coarse, feldspar-rich sand and gravel dumped off the rising Front Range in the Late Cretaceous and early Paleocene. It is the same formation that produces the petrified wood the Castle Rock area has been known for since the nineteenth century.
Arkose is a poor rock. It is soft, it crumbles, and it washes out of a road cut in a decade. Petrified wood does not. So on any given hillside east and south of town, erosion is slowly concentrating the wood on the surface while everything around it goes downstream.
Knowing it when you see it
Look for a piece that is heavier than a rock its size has any right to be, with a conchoidal, glassy break rather than a grainy one. Then look for parallel lines running the length of it — grain, not banding. If those lines converge around a knot, or if you can see the arc of a growth ring on a broken end, that is wood.
One more thing
Collect it, but collect it right
Petrified wood is a finite thing. Every piece took the end of the Cretaceous, a volcanic province and several million years of groundwater to make, and there is no second batch coming.
We collect on private land with the owner's permission, and on public land only where surface collection of common invertebrate and plant fossils is explicitly allowed and within posted limits. Rules differ sharply between BLM land, National Forest, State Trust land, county open space and city parks — and in Colorado a good deal of open space is closed to collecting entirely. Check before you fill a bucket, not after.
Vertebrate fossils are a different matter altogether and are protected on federal land. If you turn up a bone, leave it, photograph it, note the location and tell a museum.