General Geology and Soils of Boston Harbor
Geological layers of Boston Harbor - Figure 1
Distribution of soil types in Boston Harbor - Figure 2
The easiest way to understand the geology and soils of Thurston County is to understand something about the geologic history of the area. The oldest rocks in the county were deposited in the Eocene Epoch (about 38-55 million years ago) and are found mainly in southern Thurston County. At that time, Thurston County was part of a warm, shallow sea in which volcanic activity was common. This continued into the Miocene Epoch, about 10 million years ago when uplift of mountain ranges to the east caused the shallow sea to become largely filled in by sediments eroded from the mountains.
During the latter part of the Miocene Epoch and the start of the Pliocene Epoch (2-5 million years ago), the mountains to the east became eroded down to hills of gentle relief. The climate at this time was moist and somewhat warmer than today. During the Pliocene, the Cascade Mountains were uplifted into approximately their present form.
The geology, soils and land forms of Thurston County are largely the result of glacial action during the ice age that lasted from 2,000,000 years ago to 10,000 years ago (the Pleistocene Epoch). This Pleistocene ice age was a period in which the earth's climate was cooler than it is now, causing glaciers now found only in high mountainous areas to grow and move outward. These valley glaciers joined into huge continental glaciers that were thousands of feet thick. At their maximum advance, the glaciers extended to Scatter Creek south of Olympia, and to the Deschutes River in eastern Thurston County. The glaciers advanced and retreated four times during the ice age, with the last advance referred to as the Vashon glaciation. As the glaciers moved southward from British Columbia they gouged and scoured the land beneath them and picked up large amounts of sediment ranging from boulders to silt. The friction of movement caused melting of the ice at the glaciers' base, resulting in some of the sediment load being deposited as a compressed layer directly below the glacier. This formed the dense, generally impermeable material known as glacial till (also hardpan or boulder clay). As the glacier melted, the waters that flowed off it carried large amounts of silt, sand, and gravel. Coarser materials were deposited close to the glacier's edge, while sands were carried farther and deposited on the flood plains. Silt and clay were deposited mainly in lakes and marine waters. After the glacier melted, large remnant ice blocks were left on the outwash plains. These blocks were covered by younger sediments that later subsided into the voids left by the melted ice blocks, forming the numerous "kettle" lakes of the county.
The sediments deposited by these events are grouped into seven major geologic units representing the older bedrock and deposits from two glacial advances. Some of these units are aquifers, meaning they can provide water to wells or springs in usable quantities. Other finer grained silt, clay, or hardpan layers are aquitards (restrictive layers), which means they slow or restrict the passage of water. A generalized diagram of the geological layers of Thurston county is shown in Figure 1 (Copied from Figure 6 of the "Summary, Northern Thurston county Ground Water Management Plan, Thurston County, 1992).
The most recent aquifer, the Vashon Recessional Outwash, consists of sand and gravel layers deposited by streams from the melting (receding) of the most recent glacial period. This unit, which averages 25 feet in thickness, is a source of water for many shallow wells. Because it is close to the surface, this aquifer is also relatively vulnerable to contamination.
The Vashon Till lies below the Vashon Recessional Outwash. Vashon Till was deposited directly under the glacier under high pressure, and is an unsorted mixture of sand, gravel, and boulders in a compact matrix of silt and clay. Vashon Till generally has a low permeability, which makes it unsuitable as an aquifer in most cases. Most wells that do produce water from till have low yields, particularly during the late summer and fall months. Till can also retard the downward percolation of water or act as a confining layer for lower artesian ground water. A typical thickness for this unit is 40 feet, but it can range from several feet to 175 feet in thickness. In the northern part of the county, the Vashon Till is sometimes overlain by a layer of glacial lake silts, called "deposits of Lake Russell". These silts and clays, like the Kitsap formation described below, were deposited in a lake formed when the glacier dammed the northern outlet of Puget Sound.
Vashon Advance Outwash is the third major geologic unit. It consists of layers of stratified sand and gravel deposited by glacial melt waters at the front and sides of the advancing glacier. The Advance Outwash is typically 30 feet thick, but can be as much as 135 feet thick.
The Kitsap Formation underlies the Vashon Advance Outwash. This unit contains layers of clay and silt, with scattered thinner layers of sand, gravel, and peat. It is generally about 55 feet thick, but locally is up to 150 feet thick. It is unimportant as an aquifer in Thurston County, since it is ,relatively impermeable. It does, however, play a significant part in the occurrence of ground water underlying the Peninsular Area and northern- most Prairie Area in that it confines water in the underlying formation at some places and gives it a slight artesian head. In other places the Kitsap Formation effectively retards the downward percolation of water, thereby causing storage of large volumes of water in the overlying deposits of the Vashon Advance.
The fifth major geologic unit was described in older reports as the Salmon Springs Drift. More recent geologic research has shown that this unit is older than formerly thought, and is a separate unit from the true Salmon Springs Drift found in other parts of western Washington. The USGS refers to this unit in their recent studies as simply Qc, and describes it as deposits of the penultimate (second-most-recent) glaciation. This penultimate drift is a significant aquifer that averages 30 feet in thickness, and can range up to 220 feet thick. There is very limited information available on the geology below the penultimate drift, known as the Undifferentiated Deposits. The well driller's reports of the few wells finished in this unit show a sequence of unconsolidated clays, silts, sand, and gravel. The lowest and oldest geologic unit found under Thurston County consists , of sandstones with interbedded siltstone layers deposited during the Tertiary Period (2-63 million years ago). Dark, fine-grained volcanic rocks (basalt and andesite) are also common. Together these rocks form the Tertiary bedrock unit.
The geologic units described above form the complete geologic sequence in Thurston County but not all units are present in all parts of Thurston County. The layers are not uniform either in thickness or in composition and may have interbedded layers of different composition. Layers may also have become tilted due to past geologic events.
Combining the underlying geology with slope provides a basis for description of the soils in Boston Harbor. Figure 2 shows the distribution of soil types in Boston Harbor. It is from figure III-1 of the "Boston Harbor Wastewater Facilities Planning Study", 1986, by R.W. Beck and Associates.
Article from the Final Plan, Northern Thurston County, Ground Water Management Plan, 1992