Museum Of Anthropology

Granite Point (45WT41)

Geology Continued »

Geologic History and Stratigraphic Sequence

 

Geologic History and Natural Environment

The region surrounding Granite Point was formed through deep basaltic lava flows which were later overlaid with loess. The Granite Point locality is described as resting atop a “prominent grandiorite spur of pre-Miocene age jutting out above the Snake River below the Wawawai Canyon” (Reid 1991) and is located approximately two miles downstream from the historic town of Wawawai. Native vegetation was a semi-arid steppe habitat with various grasses. The rain shadow effect from the Cascade mountain range has created a modern climate with comparatively mild winters and cool summers. The region receives an average of 20 inches precipitation annually, primarily in the form of snow during the winter months, with less annual temperature variation than may be expected for the region.

 

Geologic Sequence

Sediment stratigraphy provides a chronology of natural and human activity at the site. On its own, sediment stratigraphy it is only able to relatively date the chronology of events in terms of itself.  However, radiocarbon dating of remains collected from sediment layers and the identification of specific events, like volcanic eruption, that occurred at a known time can evolve the stratigraphic chronology into a temporal sequence. Leonhardy (1970:1) stated that the purpose of the salvage excavation at Granite Point was to “document the existence of separate and discrete archaeological units in a site typical of the Lower Snake River Region of the Southern Plateau”. At the time of Granite Point’s excavation, this emphasis on establishing a temporal and cultural sequence at individual sites that could then feed into the creation of regional sequences was a common research trend among American archaeologists.

In order to create a comprehensive timeline, the 1967 excavation supplemented the sediment stratigraphy recorded from the excavation units with the stratigraphic records of 25 additional test pits that were excavated specifically for their stratigraphy. At conclusion of the field season, the field data collected at these separate exposures were used to construct a tentative geological sequence. Unfortunately, this tentative sequence was unable to fully account for some stratigraphic unconformities and correlations, requiring additional work during the 1968 excavation to complete Granite Point’s geological chronology. The 1968 excavation worked to rectify the geological sequence proposed in 1967 through the correlation of 7 key stratigraphic units: Area A, Area B, Area C, Stratigraphic Section 1, Stratigraphic Section 2, Stratigraphic Section 3, Stratigraphic Section 7. These stratigraphic correlations were made based on the sediment’s physical characteristics, relative topography and stratigraphic position and elevation, relationship to the volcanic ash horizon marker, and relationship to the soil stratigraphy horizon markers. Efforts in the field were followed by additional laboratory sediment analysis that enabled the formation of a generalized reconstruction of the geological sequence at Granite Point.

 

Geologic Sequence Basic Texture Max. Thickness
Historic Deposits/Modern Soil    
Thin Colluvium Fine sandy loam to course sand, grayish brown to dark brown 20cm
Post-Ash Aeolian Sands Very fine to medium sandy loam, pale brown to dark grayish brown 1.9cm
Later Floodplains Very fine sandy loam, light brownish gray 1.6m
Pale Loess Very fine sand to silt loam, light gray to pale brown 1.25m
Volcanic Ash Very fine sand to silt, light gray to white 65cm
Pre-Ash Aeolian Sands Very fine sandy loam, very dark brown 60cm
Early Floodplain Very fine sandy to silt loam, yellowish brown to dark brown >2m
Gravel Bar and Alluvial Fans Basalt boulder and light brownish gray granule; silt loam  
Boulder Gravel Subangular basalt boulders as much as one meter maximum in diameter  

 

Correlation of Stratigraphic Units in Seven Key Stratigraphic Sections (Leonhardy 1970, Fig. 27)

 

Radiocarbon Dating

Six samples were submitted for radiocarbon dating. Adequate samples of charcoal were not available in the earlier deposits and shell was used in its absense. Unfortunately, all of the shell dates are aberrant or contradictory.


Area Stratum Association C-14 Dates Sample Type Sample
A Stratum 1 Feature 32 955 +/- 155 B.P. Charcoal WSU-666
A Stratum 3C Feature 36 2,440 +/- 170 B.P. Charcoal WSU-667
B 60-80cm below Stratum 2 - Stratum 3 boundary   3,075 +/- 160 B.P Charcoal WSU-665
A 20cm below Stratum5 - Stratum6 boundary Feature 33 5,145 +/- 200 B.P. Shell (Margaritifera falcata) WSU-668*
A Stratum5 - Stratum 6 boundary   5,980 +/- 190 B.P. Shell (Margaritifera falcata) WSU-529*
C Upper 20cm of cobble gravel Unit 1 14,100 +/- 1,160 B.P. Shell (Margaritifera falcata) WSU-870**

Radiocarbon Dates from 45WT41 (Leonhardy 1970, Table 24)

* WSU-668 was submitted as a check on WSU-529, with which it conflicts. WSU-668 is considered the more reasonable date. ** Date considered to be far too early and conflicts with the geological estimate of the age of the gravels. This date was discarded.

 

Geology Continued »

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