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7A <br /> Page 36 <br /> Dr.Daniel Ponti Mrry 1 S,2009 <br /> Page 3 C0813 <br /> sloping contact befiween Unit D and Unit C, and fhe steeply sloping canta�t between <br /> Unit C and Unit B: <br /> � The Unit D-Unit C contact is a non-natural feature, in which fill (Unit C} was <br /> plaGed up against the exposed,steep-sided�dge of Unit D. The norfhern port�on of Unit <br /> D must have been removed prior to fil.ling with Unif C. SeveraJ, scenarios for the <br /> removal of Unit D aze possible,including nahzral creekbank erosion,natural landsliding, <br /> or human removaJ.(excavation}. Scour by the creek is a Iess likely'scenario,because the <br /> exposed cvntact is lacafed weli south of the current channel, and fhe chaxunel is <br /> m.igrating nurthward. Erosion of a portian of the creekbank is possible,and may in fact � <br /> be a. contributirtg facfor fio landsliding at this locafion. Human rem.oval of the entire <br /> , Unit D strata from�he basement foundation to the creek is even less Iikely because oE th,e <br /> large voiume of material that is missing from the secti.on. Certaix�ty, the complete <br /> removal of Unit D, from the halfway point in the trench to the margin of the creek <br /> would not have been needed for consiiuction of the b�sement. However, it is <br /> reasonable t�af fihe basement location was se�ected due to favorable fiopographic� <br /> cvnditions(reiatively flat bench resulting from prior removal of Unit D),and that minor <br /> modifications may have been made to facilitate construcfiion. ftegardl�s,fhe steep and <br /> irregular nafiure of�he cant�ct represen�,s�he upslope limit of the basement wail backfitl. <br /> Creekbank siumping (iandsliding} is a plausible explana#ion for the removal of <br /> Unz�D and also explains the morpholagy of the creek bank, and the narrowin.g of the <br /> creek channel at this locatian. The upslope edge of fhe landslide wauld lie somewhere <br /> i.xt�e area now filled wi�h Urut C. <br /> The Unit C-Unit B contact as also a sbeeply sloping, irregular contact, pxobably <br /> formed and modified simil.axly as the Unit D-Unit C contact,but with the younger unit <br /> identified as a natural deposit. In previous correspondence,BAGG had inferpreted the <br /> downsiope portion o£Lot 4#o be a mound of fill. However,the trench has led BAGG fo <br /> revise that int�rpretation,and they now identify the downslope material as a"probable" <br /> si�'eam deposit. <br /> BAGG points to the lack of slickensided surfaces and obvious shear sur£aces as <br /> indications of a lack of landsliding. A.lthough su�h features are useful in documenting <br /> the presence�of landsliding that has experienced multiple or significant displacemen�s, <br /> fhey are nol nonmally foxmed in shallow slumps iz� pooriy consolidated alluvial <br /> sedaments, and should not be expected to be found at this location. First, as explained <br /> above, the landslide scaxp is no ionger present due to subsequent human disturbance. <br /> I-iowever, if it were present, the scaxp would likely oniy be iden�ified as a steep <br /> escarpment�i.f leff unfilied),or as a thin confact be€ween the materiai tha#had not faiied, <br /> and the newer ma#e�rial deposited since the tinne+of failure. The lack a£shearing in the <br /> pull-apart regi.an of an alluvial slump should n.ot be viewed as indications that no <br /> Iartdslide has taken place. It is possibl.e that sheariz�g may have occurred.along the�ase <br /> �,�...w�.�....�.�u....��......._..�.....�..�.�_..�.�...n.w...�......�r,._..,..����,�.,...�..��_.___,..��._.......,.w....��.�,���.�....��....MW.���������I�`��..���.�;.. <br />