06 16 20 PWA_02_02 Exh2 - IS-MND

Home Browse Search

INITIAL STUDY/MITIGATED NEGATIVE DECLARATION FAIRVIEw BRIDGE REPLACEMENT AND STREET IMPROVEMENTS w MAY 2020 (9TH STREET TO 16TH STREET) PROJECT J" SANTA ANA. CALIFORNIA streams. These deposits accumulated along river and stream channels as floods and debris flows carried sediment down from higher elevations. The size, color, and types of clasts in these deposits are dependent on the local bedrock from which they were derived, with boulder -size clasts more common closer to the mountains and in areas prone to flash floods. These deposits are mapped along the Santa Ana River channel in the Project area. Although Holocene deposits can contain remains of plants and animals, only those from the middle to early Holocene (4,200 to 11,700 years ago) are considered scientifically important. Older deposits that may contain scientifically important fossils may be encountered at undetermined depths below these late Holocene deposits. Therefore, the Very Young Wash Deposits are considered to have low paleontological sensitivity. Young Alluvial Fan Deposits, which are Holocene to late Pleistocene in age (less than 126,000 years ago), consist of unconsolidated gravel, sand, and silt with occasional cobbles and boulders near mountain fronts. These sediments were deposited by flooding streams and debris flows coming down from higher elevations and generally form a fan or lobe shape at the base of hills and mountains. As noted above, only fossils from the middle to early Holocene (4,200 to 11,700 years ago) are considered scientifically important. These Holocene deposits overlie older Pleistocene deposits, which have produced scientifically important fossils elsewhere in the region. These older deposits span the end of the Rancholabrean North American Land Mammal Age (NALMA), which dates from 11,000 to 240,000 years ago and was named for the Rancho La Brea fossil site in central Los Angeles. The presence of Bison defines the beginning of the Rancholabrean NALMA, but fossils from this time also include other large and small mammals, reptiles, fish, invertebrates, and plants. There is a potential to find these types of fossils in the older sediments of this geologic unit, which may be encountered below a depth of approximately 10 ft. Therefore, these deposits are assigned low paleontological sensitivity from the surface to a depth of 10 ft and high sensitivity below that mark. According to the locality search conducted by the LACM, there are no known fossil localities within the boundaries of the proposed Project. The LACM reports that the Project area is underlain by deposits of younger Quaternary alluvium overlying older Quaternary alluvium (i.e., Young Alluvial Fan Deposits). The museum notes that these deposits typically do not contain scientifically significant fossils in the uppermost layers however; they may produce important fossils at depth. The closest vertebrate locality in these older Quaternary deposits is LACM 1339, south-southwest of the Project area near the top of the bluffs along Adams Avenue in Costa Mesa. This locality produced a specimen of horse (Equus) at a depth of 43 feet below the street. The next closest locality is LACM 2032, northeast of the Project area near the intersection of Mission Road and Daly Street. That locality yielded specimens of mammoth (Mammuthus) and camel (Camelidae) at a depth of 15 ft below the top of the bluff. Locality LACM 4943, which is located northeast of the Project area near the intersection of Glassell Street and Fletcher Avenue in Orange, produced a specimen of horse (Equus) at a depth of 8 to 10 ft below the surface. The LACM believes that shallow excavations in the Young Alluvial Fan Deposits in the Project area are unlikely to encounter any scientifically important vertebrate fossils. However, the museum notes that deeper excavations into these deposits may encounter scientifically significant vertebrate remains and should be monitored to recover those remains. A copy of the letter describing the locality search results from the LACM is provided in Appendix A. R:\WKE1702\ISMND\Final ISMND\Fairview Street Bridge Final ISMND.docx (05/26/20) 3-39