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7A
<br /> Page 35
<br /> Dr.Daniel Ponti May 15,2009
<br /> Page 2 �08�3
<br /> BAGG's trench log depi.cts 5 different geoiogi�un%ts(designated A through E).The
<br /> oldest wvt (E} was exposed along the lower foot fhe trench, and the youngest unit (A)
<br /> � for�ns a portian of�he ground �u.rface. Fntervening uziits�B, C and D are discontinuous
<br /> stra�a that are in high--angle contac�with each other. The trench exposures represent a
<br /> reIatively recent hzstory of alluviai deposiiion, remaval by erosion a�ndJor landsliding,
<br /> human disturbanc� and filling, and subsequent stream deposition. In addition to the
<br /> trench log, we reviewed and correlated �te units identified in Boring B-3., which was
<br /> previously drilled about d feet from th�trench locaf�on.
<br /> iInit E is underlain by a slightly oIder deposi#thaf was encoun#ered in Baring B-1
<br /> (and other borings on fhe prape�f�rj,but beiow the depth of the tr�flc'�excavation. V1le
<br /> refer fo th�Iowermost strata as tlnit F,which is a dense,caarse-grained deposit. Unit E
<br /> is similar in degree of consolidatian to fhe underlying Uni� F, but consists of finer-
<br /> grained alluvial sediments. Unit F (roarse) sediments were deposited by a braided
<br /> stream channe�, and Llni� E (finer-grained} sediments are �ood overbank deposi�s.
<br /> Placed in the context of Bay Area geologic history,Uni�s E and F were deposited as part
<br /> of a progxading allu�rial fan prior to m�dern entrerichment of Cordilleras�reek,and are
<br /> probably early Holocene in age.
<br /> ilnit D is considerably younger thart Unit E, based on a Inwer degr�e of
<br /> consolidation, as record�d in Boring B-1 (and other borings in the area). Unif D was
<br /> likely deposited during overbank flooding by the modern stream, and is the uppexmost
<br /> alluviai strata present acrass the praperty. The trench log shows i-hat Unit D ends
<br /> abruptly at about the ha3.fway poinf in the trench,indi�ating fhaf the creek--side porti.on
<br /> of Uni�D has been removed sin�e deposition.
<br /> I.�nit C is identified as human placed fili. N�axby Borulg B-1 does not describe any .
<br /> materials that appear to be equi�aIent to Unit C. Unit C is iikely only Iocalized fill
<br /> assoo.ated with the cvnstructian and backfilling of the basemen� structure loca�ed
<br /> adjacent to fhe trench.
<br /> LI'nit B is identified as a probable natuxal sfream deposit that�corporates human
<br /> rultural de�ritus in horizontal Iayers. Unit B mu�t be younger than Unit C, due to the
<br /> north-sloping contact between the two units. If the basement structure was cvnstruc�ed
<br /> before T930 (a�cording to BAGG,januaxy 2b,2a�9}then LTnit B represen�s a very young
<br /> stre�m deposit.
<br /> IXnti-A is interpreted by BAGG to be a thin(I foot and less)layer of h.uman-pla�ed
<br /> fill,that covers�he ground over ihe north portion of the�rench.
<br /> Ex�iana�.an of hi€ h-angle ronta�fs iden�ified in tx�nc.h
<br /> Alluvi.ai sedimenfis �re deposifed in horizontal Iayers, as flood waters rise and
<br /> reGede. Consequen�ly, contacts that are not horizontal must be exp�ained by pos�-
<br /> d.eposi#ional evenfs. hTvn-�hor.izontai con#acts observed`ut fhe#ren�h.include: the steepiy
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