Geomorphic Setting
The Gee Creek archaeological sites are found within the Portland Basin geologic setting. The Portland Basin is a topographic lowland positioned between the Tualatin Mountains to the west, the Lewis River to the north, the foothills of the Cascade Range to the east, and the Clackamas River to the south. The Columbia River flows through the Portland Basin to the south and west of the Gee Creek sites.
The Gee Creek sites are located within the Gee Creek watershed, near a channelized tributary stream of Gee Creek. Gee Creek is a perennial tributary of the Columbia River, draining in a northwesterly direction from the gently sloping hills of the west-central portion of Clark County, through the city of Ridgefield, and into the lowland lakes that line the Columbia River in the area of the Ridgefield National Wildlife Refuge.
The sediments that underlie the Gee Creek sites include materials deposited during catastrophic flooding that occurred at the end of last ice age, between around 19,000 and 12,000 years ago. During that time, the Clark Fork River in Montana was periodically blocked by glacial ice, resulting in the formation of glacial Lake Missoula. Episodic failure and breaching of these ice dams caused the release of catastrophic floodwaters into eastern Washington and the Columbia River Basin. These Missoula Floods (also called the Bretz or Spokane floods), scoured the landscape, removing topsoil and sediment. Left behind on the land were deposits of sediment ranging in size from boulders to silts and clays. |
Quaternary surficial geology of the Portland Basin
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A constriction in the Columbia River Valley is located downstream of the Gee Creek sites area at Kalama Gap. During many of the individual flood events temporary ponding occurred upriver, in the Portland Basin and even further to the south into the Willamette Valley, due to this constriction. The ponded water reached elevations of 400 to 500 feet high in the Portland Basin. Recession of the flood waters and drainage of the ponded basin left behind bedded fine-grained sediment, including fine sands, silts, and clays.
Reworking of the fine-grained Missoula Flood deposits by wind action occurred after the floods. The landscape would have been stripped of vegetation by the force of the floods, then buried in meters of alluvial sediment. Sediment deposited in areas further away from the Columbia River or at higher elevations would have been finer grained. These finer-grained sediments (sadns, silts, and clays) would have been available for transport and redeposition by both water and wind. Even after vegetation was reestablished, subsequent episodes of sediment erosion and redeposition would have occurred during periods when vegetation cover was depleted, such as after regional wildfires, after a flood, or during periods of extreme drought.
The artifacts and features discovered at the Gee Creek sites were found in these fine-grained, redeposited sediments. Previous investigations of archaeological sites in the general vicinity of Gee Creek have found deeply buried archaeological materials in non-alluvial settings. Some of these deeply buried deposits have been dismissed as the result of post-depositional disturbances, such as displacement by tree roots or animals living and burrowing in the soil. However, the investigations at the Gee Creek sites indicate that sediments geologically mapped as Pleistocene-aged Missoula Flood deposits are mantled by younger, Holocene-aged wind-blown deposits and contain important buried archaeological deposits dating to around 5,000 years later than the last major catastrophic flood event.
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Climate and Vegetation
At the end of the last ice age (around 12,000 years ago), regional climate in the vicinity of the Gee Creek sites was much cooler and drier than today. The glaciers capping the mountains to the east of the sites were retreating, but the region continued to experience cold conditions where pine, spruce, and hemlock trees grew in a subalpine parkland tundra. As global temperatures began to rise at the end of the ice age, the glaciers melted from surrounding mountain peaks, the region warmed, and less arid conditions prevailed.
The earliest evidence of humans occupying the Gee Creek sites dates to around 8,000 years ago. Glaciers had long since retreated from the region by this time, except for the highest peaks in the Cascade Mountains, and the climate had become warmer and drier than at present. Summer droughts frequently struck the area, coinciding with an increase in regional forest fires. Douglas-fir trees, alders, poplars, and hemlock trees populated the landscape.
As temperatures continued to rise during the middle Holocene (from 8,000 to 4,500 years ago) and conditions became drier, tree cover became increasingly sparse, while grasses and other herbaceous vegetation began to dominate the landscape. This oak savannah setting of open-canopied woods and grassy slopes supported many varieties of nuts, seeds, root crops (including camas), and other plant food. Acorns from oak trees provided an important source of protein for humans occupying the Gee Creek sites during this time period. Residue analysis on artifacts from the sites suggest that nuts represented an important local food resource.
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Oak trees in prairie setting
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| A variety of animals would have inhabited these vast prairie landscapes. People living at the Gee Creek sites constructed large earth ovens to process these vegetable and animal resources. Fats and oils from nuts and seeds were detected on rocks found in earth ovens that dated to nearly 8,000 years ago. Residue from lilies (possibly camas) was detected on rocks from one of the earth ovens. Organic residue from large game animals, birds, and possibly turtle were also detected on stone tools and fire-modified rock dating to this early time period. The presence of fish oil residue on materials from one of these oven features suggests that Gee Creek or its nearby tributaries supported fish populations. |
Charred camas bulb from an earth oven feature found at an archaeological site in western Washington (45LE611), near to the Gee Creek sites (scale bar = 2 cm). This earth oven was nearly 3,000 years old.
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Camas field in southwestern Washington
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From 4,500 years ago to the present, the regional climate shifted to cooler and wetter conditions. Grasslands that had dominated the landscape retreated as forests expanded into the area. Douglas-fir trees, western hemlocks, red alder, and ash trees flourished. Cupressaceae (likely western redcedar) and alder occupied riparian and poorly drained portions of the landscape, including areas surrounding the Gee Creek sites. The forest understory was dominated by brackenfern, a resource that ethnographic accounts indicate was used by many Native Indian groups for food. People living in the area at this time would have experienced generally temperate and humid conditions, much like today.
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Douglas-fir and western redcedar trees surrounded an open, grassy field at the time that the Gee Creek sites were discovered
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| General climate, vegetation, and fire histories for the Gee Creek project area were compiled based on data collected from Battle Ground Lake, Washington, and other regional locations. Click here to view a summary graphic. |
References:
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