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April 7, 2005 <br /> Mr. Don Lc��in <br /> L&A Job No. 3A037 <br /> Pagc 3 <br /> Thc gcology of lhc Pugcl Sound Lowland has bccn modificd Uy thc advancc and retrcat of <br /> scvcral glacicrs in thc past and subscqucnt dcposits and crosion. Thc latest glacicr advanccd lo <br /> thc Pugct Sound Lowland is rcfcrred to as thc Vashon Stadc of thc �rascr Glacialion, which has <br /> occurrcd during thc latcr stagcs of thc Plcistocenc Lpoch and retrcatcd from thc rcgion somc <br /> 14,500 ycars ago. <br /> 7'hc Vashon till soil unit is dcposilcd dirccUy by glacial icc during thc most rcccnt glacial period <br /> as it ach;mced ovcr an crodcd, irregular surfacc of oldcr fonn:uions and scdimcnls. It is a vcry <br /> dcnsc mi�turc of unsortcd day, silt, sand, gravcl, and scattcrcd cobblcs and bouldcrs, o(lcn <br /> rcfcrrcd lo as "hard pan". Thc Vashon till ovcr thc top h��o to four fccl is nonnally wcathcrcd to a <br /> mcdium-densc statc, anJ is modcratcly pernicablc and compressiblc. Thc undcrlying fresh till is <br /> vciy dcnsc and is practically impervious to stonnwatcr infiltration. Thc Gcsh lill has thc strcnglh <br /> of low-gradc concrctc and can stand in a stccp natural or cut slopc for a long period. If remains <br /> unJisturbcd and wcll-draincd, thc fi•csh till can pro��idc cxccllcnt found.�u��, 5��npor� "-��n i��u� <br /> sclticmcnt c�pcctcd. <br /> 'I�hc a�l�ancc uut��:ish soil tmit undrrll'in� thc �'ashun till is cotnposcd of stratificd sand and <br /> gravcl ���ilh minor amounts of silt and clay, dcpositcd by thc mcltwatcr of ach�ancing glacial icc. <br /> Duc to thcir gcncrally granular composition, thc ach�ancc ouhvash dcposils arc of modcratcly <br /> high pcnncabiliry and gcncrally Jrains f:tirly ���cll. Thc a�h�ancc ouh��ash dcposits arc glacially <br /> ovcrriddcn and arc gcncr:illy dcnsc to ��cry Jcnsc in its natural, undisturbcd slatc. �.��n� a,� �on z <br /> to 4 fcct ���hcrc c�poscd on slopcs which may bc «�catlurcd to a loosc to mcditun-dcnsc statc. <br /> l'hc ad��;mcc outa�ash dcposits can stand in slccp cuts or natural slopcs for c�tcndcd period �vhcn <br /> undisturbrd. \Vhcrc cxposcd on slopcs ��•ith poor ��cgctation covcr and subjcct to stomi nmoff or <br /> ground���atcr sccpagc, thc ade.mcc out���ash dcposits can bc gradually crodcd and may slough and <br /> \ <br /> LIU & ASSOCIATES, INC. � ��\� ) <br /> �_� <br />