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City of Pleasanton—Hidden Canyon Residences and Preserve Draft EIR Geology, Soils, and Seismicity FirstCarbon Solutions 3.6-1 https://adecinnovations.sharepoint.com/sites/PublicationsSite/Shared Documents/Publications/Client (PN-JN)/2148/21480016/EIR/3 - Draft EIR/21480016 Sec03-06 Geology.docx 3.6 - Geology, Soils, and Seismicity 3.6.1 - Introduction This section describes existing conditions related to geology, soils, and seismicity in the region and project area as well as the relevant regulatory framework. This section also evaluates the possible impacts related to geology and soils that could result from project implementation. Information included in this section is based, in part, on the Updated Preliminary Geotechnical & Geologic Report prepared by Cornerstone Earth Group dated March 31, 2020, University of California Museum of Paleontology (UCMP) paleontological records search request and an Updated Geotechnical Review prepared by Ninyo & Moore dated February 12, 2025, which are included in Appendix E as well as the City of Pleasanton General Plan, Association of Bay Area Governments (ABAG) Resilience Program, and the United States Geological Survey (USGS). During the Environmental Impact Report (EIR) scoping period, one comment was received related to geology and soils. • General objections to grading and building on protected slopes. 3.6.2 - Environmental Setting Geologic Setting Alameda County Alameda County is located in the western Diablo Range within the Coast Ranges geomorphic province of California. The Coast Ranges have a complex geological history characterized by Late Tertiary folding and faulting that has resulted in a series of northwest trending mountain ranges with intervening alluvial valleys. Bedrock in the Coast Ranges consists of igneous, metamorphic and sedimentary rocks that range in age from Jurassic to Pleistocene. The present physiography and geology of the Coast Ranges are the result of deformation and deposition along the tectonic boundary between the North American plate and the Pacific plate. Plate boundary fault movements are largely concentrated along the well-known fault zones, which in the area include the San Andreas Fault, Hayward Fault, and Calaveras Fault, as well as other lesser-order faults. Project Site The project site is located in unincorporated Alameda County, within the northwestern portion of the City of Pleasanton’s urban growth boundary near Devany Canyon. The project site consists of a series of hills and knolls, sloping down to a canyon bordering the southeast site boundary. The highest elevation on the project site is approximately 774 feet above mean sea level (AMSL) at the ridgeline in the proposed East Bay Regional Park District (EBRPD) open space area, and the lowest elevation is approximately 438 feet AMSL along Devany Creek. Exhibit 2-8, Slope Analysis and Grading Plan, identified areas where slopes exceed 25 percent, and how the proposed site plan is situated to largely avoid them. As noted in the geotechnical report prepared by Cornerstone Earth Group, the geologic units mapped as trending through the site include Miocene-age marine sedimentary rocks incorporated with sandstone, siltstone, and siliceous shale folded and transected by inactive northwest trending