Malheur National Forest Geology and Rock Types

Rocks in the geologic formations on the Malheur National Forest lands range from about 10 million to over 250 million years old. They include marine and other sediments, contact and low-grade regional metamorphic rocks, and igneous volcanic and intrusive rocks. Some typical types include:

Igneous

Volcanic - obisian, pumice, cindars, dacite, rhyodacite, rhyolite, andesite, porphyritic andesite, basalt, diktytaxitic and amygdaloidal basalts, breccias and conglomerstes of mudflow and other origins, welded and non-welded tuff deposits

Intrusive - quartz-biotite diorites, other siliceous dikes

Metamorphic

An ophiolite complex with dunite, harzburgite, pyroxenite, meta-gabbros, peridotite, serpentinite and chromite (ultra-mafics), and other metamorphic formations which include argillitic schists, phyllites, greenstones, greenschists, serepentine, and meta-gabros, -dacites, and -andesites

Sedimentary

shale, siltstone, mudstone, sandstone, graywacke, limestone, chert, pebble conglomerates, volcanic ash, and stuff deposits (waterlaid)

Some associated minerals and rocks include quartz, calcite, garnets, and various types of agate, jasper, opalite, etcetera.

Fossils

The only known fossil area on the Forest is the Rosebud Fossil Beds, which are located in a roadcut in Forest Service Road 6370 on the Bear Valley Ranger District. This site has a variety of marine fossils and is accessable with a passenger car.

There have been some Tempskya (fossilized ferns) found around the Greenhorn area, north of the Long Creek Ranger District, in the Clarno formation, but these are very rare, in an area of many mining claims, and people who have found them won't reveal where.

There is ample opportunity to visit fossil sites at the John Day Fossil Beds National Monument, adjacent to the Forest.

Petrified Wood

There are a couple of areas on the Forest where petrified wood has been found. Two of the most notable are:

1. Along Forest Service Road 17, Burns Ranger District, in roadcuts/ditchline in areas between 5 and 10 miles east of Highway 395. Found in deep deposits of volcanic ash; also opalite and jasper occurs locally.
2. On the surface of conglomerate and breccia outcrops of the Clarno formation. It's recommended to do your looking anywhere these outcrops occur above and along the mainstream or east and west forks of Lick Creek (Roads 3670 and 3675) on the Long Creek District. There are also very good sites to find a variety of quartz, quartz crystals, calcite, and banded agates and geode type quartz deposits. Access off of the main roads is not genereally suitable for passenger cars.