Soils of the Puget Sound Area - Puyallup Research & Extension ...

Soils of the Puget SoundAreaCraig Cogger, Ph.D.,Washington State UniversityCooperative Extension Soil SpecialistWSU Puyallup Research & ExtensionCenter

Soils of the Puget Sound AreaUnderstanding our soils and landscapes can helpus learn how to live on the land without abusingit. As we have developed more land in the PugetSound region, we have become more aware of theeffects of development on the environment. Usingour knowledge of local soils can help us becomebetter caretakers of the land and water, so that wecan maintain them for future generations.

Okanogan CountyTo understand dd the soils in an area,soil scientists study the landscapeand the soils below the surface atdifferent places in thelandscape. The landscape in thisslide has three distinct landforms, which formed by different geologicprocesses and contain different types of soils. A glacier covered the lowerand middle areas of the landscape more than 15,000 years ago. The soilson the hummocky land in the middle developed from material left behind byglacial ice. Soils formed from material that was moved, sorted, anddeposited by water comprise the foreground, once a stream bed formeltwater from the glaciers. The mountains in the background poked abovethe glacier; there the mountain soils developed directly from the underlyingbedrock.

Carstairs Soil Profile,Mason CountyThis soil has three distinct layersor horizons, each with differentproperties. Soil scientists useinformation on the color, texture,and other properties of eachhorizon, along with landscapeinformation, to describe a soil andits uses and limitations.

Parent Materials:Glacial Volcanic Flood DepositsParent material is the geologic material from which a soil has formed. Glacial ice andmeltwater, volcanic activity, and flood water deposited the major parent materials forsoils in the Puget Sound area. Because soils in our area are young in terms ofdevelopment, their properties strongly reflect the properties of the parent materials.In addition to parent material, four other factors affect soil development. These are:• Climate• Vegetation and other organisms that live in the soil• Topography or landscape position of the site• Age of the soilIn the following slides we will see some important parent materials of the PugetSound area, and the types of soils which have developed from them.

Glacial Parent Materials:Till (ice laid)Outwash (meltwater deposits)Lacustrine (lakebed deposits)Three important types of glacial parent materialsoccur in the Puget Sound area – glacial till (depositedby ice), glacial outwash (deposited in glacialmeltwater streams), and lacustrine (glacial lakebedsediments). Each has different properties and hasformed distinct types of soils.

Carbon Glacier,Mount RainierMore than fifteen thousandyears ago, massive glacierscovered the Puget Soundarea as far south asOlympia. More than fifteen thousand years ago, massiveglaciers covered the Puget Sound area as far south asOlympia. The glacial ice was nearly one-half mile thick atSeattle and obliterated all the soils beneath it. When theglaciers melted, much of what was left behind was a mixture ofsand, silt, clay, and rocks called till, much like the material at thefront of and under the Carbon Glacier in the slide.

Carbon Glacier,Polarized FilterWhen the glaciersmove, soils androcks freezealong the glacier, and eventually they areburied within the glacier and compressed bythe overlying ice. This mixed, compactedmaterial is called basal glacial till.

Glacial Boulders,Douglas CountyA very large glacier,such as the one thatcovered the PugetLowland, A very largeglacier, such as the one that covered the PugetLowland, can move large amounts of material andsubject it to tremendous pressure. Glaciers pluckedthese boulders from the ground and moved themacross the landscape.

Glacial Till NearLake Tapps,Pierce CountyWhen the greatglaciers melted, theyleft behind thick, very dense deposits of basaltill, which you see in the slide. Basal tillcontains an unsorted mixture of materialsranging in size from microscopic clay particlesto rocks and boulders.

Shelton Soil Profile,Mason CountyThis is the profile of a soil developed from glacial till. Theupper part of the profile developed from material that was ator near the top of the glacier, where it was not under greatpressure. It also may be mixed with material moved by windor water. It is quite permeable, and water has drained freelythrough it since the glaciers melted, giving it the brownish toreddish color typical of well-drained soils. The lower part ofthe profile is the very dense, nearly impermeable basaltill. Often called hardpan, it usually appears 20 to 40 inchesbelow the surface, and extends to a depth of 10 or more feet. The soil in this slide is on aslope. Water can move downslope along the basal till, leaving the overlying soil welldrained. Soils similar to this one are common throughout the Puget Sound area, fromBellingham south to Olympia. Although used for urban and suburban development, these soilsare seldom deep enough for conventional septic systems. The basal till does provide a stablebase that is resistant to earthquakes. Glacial till soils are suitable for pasture. They are alsoused for growing row crops, but are limited by rocks and shallow depth.

Kapowsin Soil,Pierce CountyThis soil also developed fromglacial till, but it looks muchdifferent. The profile is grayerbecause the soil is wetter.Since the soil sits on a flat landscape, water thathits the hardpan has no place to go. The lowestspots on glacial till landscape collect water fromhigher ground and are usually wetlands.

Soil Shallow toBedrock, SanJuan IslandIn parts of the SanJuan Islands andsome nearby mainland areas, only a shallowlayer of glacial till covers the bedrock. Thesesoils often are too shallow for septic systemsand have little potential for agriculture orforest production.

Outwash fromCarbon Glacier,Mount RainierA second type ofparent materialsfrom glaciersis outwash. It consists primarily of sand,gravel, and rocks that meltwaters fromglaciers have moved and sorted.

Outwash Deposit,Pierce CountyGlacial outwash materials wereseparated by size as meltwatermoved them. Changes in thespeed of the flowing meltwatercaused the different bands or stratavisible in this deposit. Fastermovingwater deposited the stratacontaining mostly gravel, while slower-moving water depositedthose containing mostly sand. Often deposits of outwash aremany feet thick, similar to the one shown in this slide. Thelowest parts of outwash deposits, often saturated, can beimportant sources of groundwater (aquifers).

Everett Soil,Pierce CountyThis soil developed from gravelly glacialoutwash. The dark surface is thicker thancommonly seen, but the rusty-colored subsurface istypical. Soils of this type usually are loose, coarsetextured,and uncompacted, and water percolatesthrough them easily. They are difficult to farmbecause they often are stony and hold too littlewater for crops. In the past, much suburban growthcame to be concentrated on these soils. Gravellyoutwash soils usually do not provide muchprotection for underlying groundwater. Because ofconcerns about groundwater contamination, health departments now require largerlots and more complex septic systems in these soils. Similar soils are common inmuch of the Puget Sound area. Some outwash soils are mostly sandy with little or nogravel. They are common in Whatcom and Thurston counties, where they areimportant agricultural soils.

Lake Campbell,Skagit CountyGlaciers depositeda third type ofmaterial in the bedsof glacial lakes andmarine bays. Asthe glaciers melted, many temporary lakes formedwhere ice blocked valleys and trapped glacialmeltwater behind them. In the quiet waters of thelakes, fine-textured sediments (silt and clay) settledout, forming lacustrine parent materials.

Cloquallum Subsoil(Lacustrine Deposit),Mason CountyLacustrine soilshold morewater thanoutwash or till soils because theycontain more silt and clay. They alsoare less permeable than outwashsoils.

Whatcom Soil,Whatcom CountyThe mottled gray and brownsubsurface of this profile indicates thatit is very wet for at least part of theyear. Glacial lacustrine and marinesoils can be good agricultural soils, butseasonal wetness often limitsthem. They also are harder to cultivatethan sandier soils. Lacustrine soilsoccur in pockets throughout the Puget Sound area. They arecommon on the slopes above Puget Sound and the major rivervalleys in areas prized as view lots. However, their fine textureand susceptibility to landslides and erosion on steep slopesmakes them poorly suited for development.

Till and OutwashMixed, Key Center,Pierce CountySometimes we find amixture of parentmaterials in the sameprofile. This profile hasboth sandy outwash andcompact basal till.Evaluate soils like thiscarefully to determinehow best to use them.

Volcanic Parent Materials:Tephra (ash)Mudflow

Mount St.HelensVolcanoes also have been asource of parent material forsome western Washingtonsoils.

Mt. St. Helen’sTephra in SandDune, Grant CtyWe all know about thevolcanic ash that erupted fromMt. St. Helens. Geologistscall this "tephra." Most of theMt. St. Helens tephra blew tothe east of the CascadeMountains, where it has become part of the soils. In the sand dune in theslide above, a layer of sand has blown over the layer of tephra, preserving itintact. The Northwest has a long history of volcanic activity, but prevailingwinds have blown most of the tephra east of the Cascades. Soils in thePuget Sound area contain some tephra, primarily from the Mt. Mazama(Crater Lake) eruption that occurred more than 6800 years ago. Weatheredtephra tends to increase the water holding capacity and reduce the densityof soils.

Mudflow,Mt. St. HelensMudflows also resultfrom volcanicactivity. Massivefloods of meltedsnow and ice combined with soil, rocks, anddebris to create mudflow deposits. They lookand behave very much like basal glacial till;they are a mixture of materials, and they arecompact and often slowly permeable.

Profile of a 1980 Mt. St.Helens Mudflow DepositAlong the Toutle RiverThis 1980 Mt. St.Helens mudflow justnow is beginning toweather into soil. Itlooks very much likeglacial till parentmaterial.

Buckley Soil Profile,Pierce CountyMudflow deposits from Mt. Rainierdominate the landscape in parts ofKing and Pierce counties. Thesesoils are quite wet because theyare slowly permeable and arelocated on flat landscapes. Thethick, dark surface, combined with the mottled color pattern inthe subsurface, results from wet conditions. The wetnessseverely limits their suitability for development. They are bestsuited to pastures, woodland, and wetlands.

Alluvial ParentMaterials:Flood Deposits

Puyallup River atFlood StageFloods have provided the parent material forthe Puget Sound region’s fertile young valleysoils. Modern flood control, however, hascut the supply of sediments to many of ourvalleys.

Daffodils andMt. Rainier,Puyallup Valley,Pierce CountyThese alluvial (waterdeposited)soils are level,permeable, free of rocksand ideal for agriculture.They often have layers of different texture, resulting fromdifferent rates of flow of flood water. A high water table existsin most of our alluvial soils, making these soils better suited foragriculture than for development. In addition, they do notprovide stable support during earthquakes. Shallowgroundwater beneath these soils is vulnerable tocontamination from both agricultural and nonagriculturalsources, as occurs with the groundwater beneath glacialoutwash soils.

Minter Creek,Pierce CountyWe have many differentsoils in the Puget Soundarea, each having its ownorigins, history, andcharacteristics. Some soils are better suited for development,some for agriculture, and some are best left to serve theirnatural functions in wetlands, buffers, and wildlifehabitat. Understanding our soils is a key to environmentallysound land use and planning.