Typical nodule of obsidian found at Inman Creek type location.
The Inman or Inman Creek source consists of two geochemically-distinguishable varieties of glass, Inman Creek A and Inman Creek B. The two types almost always co-occur in geologic contexts where the glass is found, although the "A" type is more commonly found in natural deposits than the "B" type by a typical overall ratio of approximately five to one. The source was originally identified in the gravels of Inman Creek, a tributary to the Fern Ridge Reservoir (Long Tom River) and the Long Tom River in the southwestern Willamette Valley of Oregon (Davis 1970; Skinner 1983:304-320, 1986, 1991). Nodules of obsidian up to 20 cm in diameter, but more commonly less than five centimeters in diameter, are commonly found at that location.
Secondary outcrops of obsidian correlated with the Inman Creek geochemical varieties.
Obsidian correlated with the two chemical groups has since been identified in widely dispersed geologic contexts at many other western Oregon locations, including the mouth of the Siuslaw River at the Oregon Coast, the Pleasant Hill area of the southeastern Willamette Valley, the gravels of the Willamette River, and several locations in the Mt. David Douglas area in the Western Cascades (Jackson et al. 1994; Skinner 1983, 1986, 1993; Woller and Black 1983; see map above). The primary source for Inman A type appears to lie near Salt Creek Falls in the Upper Middle Fork Willamette River drainage at the southern base of Mount David Douglas; the distribution of characterized geologic samples and artifacts suggests that primary source for the B type also lies in the same area (Jackson et al. 1994). Major geologic source localities for Inman obsidian include:
- Inman Creek: Nodules of obsidian ranging in size from less than 1 cm to a maximum of well over 10 cm in diameter are common in the stream gravels of Inman Creek, a tributary to the Long Tom River that is largely covered by the waters of the Fern Ridge Reservoir for much of the year. Obsidian nodules appear to originate from a locally widespread bed of poorly sorted and moderately indurated conglomerate that dips gently towards the northeast (the floor of the Willamette Valley). This conglomerate, best exposed 2-3 m below the ground surface in the streambed and banks of Inman Creek, contains abundant rounded to subangular nodules of obsidian and other volcanic rocks set in a clay matrix. Obsidian nodules appear to be distributed throughout the conglomerate. The obsidian-bearing stratum is poorly-exposed in all locations in the Fern Ridge Reservoir area and the thickness and areal extent of the bed are not known. The color of the Inman Creek obsidian is most often a uniform black (94%), sometimes a mottled mahogany and black (3%) or gray-black to gray (3%). Both chemical varieties of glass (A and B) have been identified from samples collected at the Inman Creek exposure (Skinner 1983, 1986, 1991). The obsidian from this and all other outcrops often contains very small microscopic phenocrysts, resulting in the subtly "flawed" surface texture that can be seen in the nodule photo above.
- Salt Creek: This location, a roadcut in Highway 58 near Salt Creek Falls, appears to be very near to the primary source vent for the Inman Creek A chemical group that is widely distributed throughout central western Oregon. All samples recovered from the Salt Creek locale have been correlated with the Inman Creek A chemical group. Zones of black and gray obsidian up to about 10 cm in diameter are interspersed with tan to gray ashy pyroclastic deposits and devitrified glass nodules exposed in Highway 58 roadcuts near Salt Creek Falls. These deposits probably represent the near-vent facies of the eruptions that produced the glass. Nodules of vitreous black and gray glass are abundant in this location and from the Mt. David Douglas area located directly to the north. Obsidian from Mt. David Douglas has been correlated with both the Inman Creek A and Inman Creek B chemical types (Jackson et al. 1994; Skinner 1993). Black glassy obsidian nodules of archaeologically-usable size are found downstream from the source in the gravels of Salt Creek as far west as McCredie Hot Springs. The geochemically-identical and more durable gray variety of glass may be available farther downstream. It is likely that the obsidian found at many secondary outcrops in the Willamette Valley was transported to the west from the Salt Creek area in a lahar (mudflow).
- Siuslaw River: Nodules of black, variegated black and mahogany, and occasional solid mahogany glass are found in gravel bar deposits near the mouth of the Siuslaw River at Florence. Most nodules are less than 1 cm in diameter, although items up to 4 cm in diameter are sometimes found. Rare small nodules have also been reported from been reported from the beach to the south of the river mouth (Personal communication, Randy Roberts, Gold Beach, Oregon, 1995). Geochemical studies indicate that the obsidian from this secondary source belongs to the Inman Creek A/B chemical groups (1991). Similar small obsidian nodules and cortex flakes, along with an unusually high percentage of obsidian debitage, has been reported from the Umpqua/Eden Site (35-DO-83) located about 30 km south of the Siuslaw River source (Lyman 1991:119-21; Skinner 1987). Recent XRF analyses of some of the Umpqua/Eden Site artifacts by Northwest Research indicates that both the Inman Creek A and B chemical groups are represented.
- Other locations: Nodules of obsidian geochemically correlated with the Inman Creek source groups have also found in the gravels of the Willamette River at several locations (as far north as Oregon City), in a laharic bed exposed about 100 feet below the surface in a Eugene gravel quarry (O'Connor et al. 1997, 2001), and in valley fill deposits in the Pleasant Hill area. O'Connor et al. (2001:15) write: "A distinct feature of this deposit is abundant subrounded to subangular clasts of black obsidian having maximum diameters as great as 15 cm. The composition and location of this deposit indicates a major Cascade Range silicic eruption that produced a lahar that traveled down the McKenzie or Middle Fork Willamette Rivers. A source for this deposit is not known, but Ar-Ar incremental hrating analyses of two obsidian clasts yielded ages of 0.418 +/- 0.010 Ma and 0.426 +/- 0.004 Ma."
The overall distribution and size diameter of obsidian nodules correlated with the two geochemical sources stongly suggests that the obsidian was transported in a volcanic lahar westward down the Middle Fork Willamette River drainage from a source in the Western Cascades. Although the geographic extent and volume of the lahar that carried the glass to the Willamette Valley floor is not well known, it is likely that the obsidian in the flow was widely distributed throughout the valley as small nodules after the emplacement of the unit. The presence of small split cobbles of obsidian and an abundance of cortex flakes at many sites ranging from Eugene to near the confluence of the Willamette and Columbia rivers points to local procurement of these extensive deposits of natural glass.
Left - Inman Creek obsidian-bearing conglomerate bed location near Eugene. Right - Close-up of in situ nodule by shovel.
Alternate Names: The source has been know by a confusing aray of names including Fern Ridge, Inman (A and B), Inman Creek, Long Tom, Salt Creek (A and B), Siuslaw River, and Staley Creek. In some earlier (pre-1994) XRF studies of obsidian artifacts by BioSystems Analysis, the Inman Creek source was known as Unknown D. At one point early in trace element studies of Willamette Valley obsidian, the Inman glass was confused with obsidian from Tucker Hill, a source of obsidian located in central Oregon (Toepel and Sappington 1982).
Geologic Age: The source at Salt Creek in the Western Cascades (Inman Creek A) is mapped as Oligocene to Miocene in age by Woller and Black (1983). Ar-Ar ages of 0.418 +/- 0.010 Ma and 0.426 +/- 0.004 Ma are reported for two obsidian nodules from the Delta Sand and Gravel Company quarry in Eugene (O'Connor et al. 2001:15, Appendix 1).
Chemical Composition: The major and/or trace element composition of both varieties of glass is reported in Jackson and Davis (1993), Jackson et al. (1994), Skinner (1983:152-166, 1986), and Woller and Black (1983).
Prehistoric Use: Artifact obsidian correlated with the Inman chemical groups has been identified at many archaeological sites in the Willamette Valley and the Middle Fork Drainage of the Willamette River in the Western Cascades, as well as from a few locations at the Oregon Coast (Cheatham 1988; Skinner 1987; Skinner and Winkler 1991, 1994; Wilson 1995). Unpublished XRF studies by Northwest Research have also identified Inman Creek glass at the Umpqua/Eden Site (Oregon Coast; 35-DO-83), the Fanning Mound (Willamette Valley; Murdy and Wentz 1975; Laughlin 1943), and at other scattered sites near Dallas, Amity, and Salem (Willamette Valley). Significant quantities of Inmsn Creek artifacts have also been found at Columbia River sites both upstream and downstream from the confluence of the Willamette and Columbia rivers.
Obsidian Hydration: Comparison of obsidian hydration measurements of characterized Inman and Obsidian Cliffs artifacts from similar Willamette Valley archaeological contexts by Wilson (1995) suggest that the hydration rate for the Inman chemical groups is significantly slower than for Obsidian Cliffs (the other major source found in Willamette Valley archaeological collections). Inman obsidian use is restricted almost entirely to western Oregon. Only a few possible Inman artifacts have been identified in characterization studies of central Oregon materials and prehistoric use appears restricted almost entirely to northwest Oregon.
Looking west down the Siuslaw River towards the Pacific Ocean. Small nodules of obsidian
correlated
with the geographically widespread Inman Creek geochemical sources are found at this location.
