5700 23RD DR W 2016-01-01 MF Import

t 1 I4 C �' The geology of the project area is illustrated on Figure 2. This map was developed by inurpreting � data obtained from the subsurface exploradons completed at the site by AFSI aad from sediment exposures observed along the drainage ravines. {Exploration pit logs are included in the Appendix of this report.) � Two distinct surficial glacial units were encountend in this study. These glacial units are related , to the Vashon-age glaciation, which was th�most recent (12,000 to 15,000 years before present) I glacial event in this area (Blunt et al., 198'i'1. Older pre-Vashon sed'unents were also encountered in a few exploradon pits at the top of the slo�e adjacent to Nazbeck Creek. Alluvial and colluvial i sediments locally form a thin venecr over the glacial sediments along the base of slopes. The � presence of fill soils was inferred along the sanitary sewer pipeline. A description of the stratigraphic units encountered on the site is presented in the following � paragraphs, organized from oldest to youngest in age. I4.1 Pre-Vashon Una;fFPr .±*+arad ((�RYi: Very stiff to hard, moist, gray to reddish-brown silt was encountered in EP-7 through EP-10. The isilt was encountered at a depth of about 3-1/2 feet in EP-7 and EP-8 and ezteaded beyond the termination depth of the explorations. The top of the silt was penen�ated at a depth of about 12-1/2 � feet in EP-9 and 11 fcet in EP-10. The degree of induration and wlor was vaziabie betwan explorations, and it is possible that more than one unit was encountered. The age and geologic origin of the silt units are uncercain and have been classified as pre-Vashon undifferentiated for � this sNdy. The glacially consolidated sand and gravel deposits exposed in Narbeck Creek have also been included in this unit. + 4.2 Vachon Adv n �cw ch (Ova) � Dense, wet, gray to brown gray, medium to coarse sand witti gravel and fine to medium sand with Itl»n silty sand interbeds were exposed in several exploration pits and have bcen interpreted as Vashon advance outwash. Vashon advance outwash sand was deposited by meltwater streams emanating from an advancing glacial ice sheet. In the project area, these sediments were ; overridden by an approximate 3,500-foot thiclmess of glacial ice during the Vashon stade of the Fraser glaciation about 15,000 years ago, and consolidated to a dense state by the weight of the ice. Vashon advance outwash deposits aze exposed near ground surface over about 2/3 of the site � (Figure 2), and in EP-7 and EP-8 were underlain by pre-Vashon undiffenntiated silts. � 4,3 Vashon T nripement Till (Ovtl Vashon lodgement dll sediments aze interpreted to direcdy overlie advance oucwash sand deposits � in exploraaons in the upland areas. The lodgement till consisted of a dense to very dense, brown- gray, unsorted mixture of gravel, sand, silt and clay. Lodgement till was generally e�auntered in the southeastern poRion of the site (Figure 2). � 4 l