Name/Title
Roxy Ann WoodEntry/Object ID
2024.58.26Description
Assemblage Zone: volcanic sediments
Chemical Composition: Si O2
Crystal System: Hexagonal
Description: Family: Not Identified
Common Name: Limb Cast
Age: Oligocene
Location data: Dry creek, Roxy Ann Peak, Jackson County, Oregon, USA
Payne Cliffs Fm.
Description: Red, black, brown and yellow with growth rings.Circular pattern of agatized minerals.
Type of Fossils Present: Plants
Fracture: conchoidal
Grain Size: Fine
Hardness: 7 Quartz
Lithofacies: volcaniclastic
Luster: Glassy
Occurrence: During the Oligcene (23 to 34 million year ago), standing forests were often destroyed by volcanic eruptions and then buried by falling volcanic ash and /or by mudflows of volcanic ash, called lahars. In some cases these destroyed forests were exposed to atmospheric oxygen because the wood was on the surface or because the layer of volcanic material enclosing the wood had dried out, which allowed microbes to attack the decaying wood. Subsequent volcanic eruption covered this material with another thick layer of volcanic ash or mudflow deposits; then silica-rich fluids derived from the surrounding volcanic ash replaced the partially rotted wood with amorphous silica, which later re-crystallized into chalcedony.
These layers of volcanic sediment and wood fragments were buried by thousands of feet of additional material and transformed by heat, pressure, and fluids. In the Late Miocene (8 to 10 million years ago), these buried layers were uplifted, which over time allowed erosion to produce the modern landscape.
Pressure: Moderate
Rock Color: Medium
Rock Origin: Post-depositional
Rock Type: Sedimentary
Specific Gravity: 2.6
Surface Process: Not apparent
Streak: white
Temperature: Low to moderate
Texture: Mircrocrystalline
Variety: "Roxy Ann Wood " is composed of a mélange of petrified wood, chalcedony and jasper, with in fillings of volcanic ash and microcrystalline quartz. The specimens show woody texture on their outer surfaces, they are not true limb casts. According to geologist Marty Norris, the conditions necessary formation of limb casts were present, but these specimens were shattered, splintered and then began to decay before their transformation into petrified wood, and so they are "wood cast."
Mr. Norris notes that some "Roxy Ann Wood " specimens show compressional deformation and other have fractures in a directional pattern, both of which suggest that there was downward pressure from the weight of overburden and or lateral pressure from crustal movements before the amorphous silica replaced the wood fragments. The growth of quartz crystals on the outer surfaces of some specimens, which formed after the wood was replaced by silica and fractured, supports the conclusion that there were several episodes of fluid migration through the sediments.Other Names
Name Type
Previous Accession/Collection NumberOther Name
Doug Foster Acc.#F302