Evidence of a north projecting ancient boulder thalweg in latest Quaternary deposits of the Santa Ana River, Orange County, California

The shallow sub-surface geology within north central Orange County, California, was studied through existing logs of wells in order to identify the northernmost location of an ancient shoreline and to characterize the subsurface geology of the ancient fan-delta complex of the Santa Ana River. The shoreline was determined to be in the relatively same location as the 405 freeway and follows the same northwest to southeast pathway as the freeway through Orange County. The location of the ancient shoreline was determined based on well log descriptions from wells located south of the study area that demonstrated changes in stratigraphic continuity and abrupt fossil/grain size changes at an approximate depth of 200 feet below ground surface. This shoreline is interpreted to be the Stage 3 shoreline. Uniform sand deposits observed in stratigraphic profile diagrams generated within the study area provided evidence that the ancient shoreline extended into the city of Anaheim, north of the forebay/pressure area boundary, which is located in close proximity and runs parallel to the 5 freeway. A significant discovery made from stratigraphic data from numerous well logs located within the study area is that of a linear unit of boulders ranging in thickness from 3--61 m (10--200 ft), containing a maximum width of 1,829 m (6,000 ft), that extends in a north/south direction from near Yorba Linda southwards to Garden Grove. This string of boulders is interpreted to represent an ancient thalweg that traveled through central Orange County on its course to the Pacific Ocean. Most significantly, this boulder channel extends north to northeast past the northern limit of the current course of the Santa Ana River and contains boulder size material not seen in the Santa Ana River, either in the Santa Ana River Valley or immediately upstream of Prado Dam.; This paleochannel is most likely the course of a stream or confluence of streams that was generated in the San Gabriel Mountains prior to uplift of the Chino-Puente-Coyote Hills. The course of the ancient channel is fairly straight and shallows at an angle of approximately 0.65 degrees as it is traced to the north. The boulder train ends at Yorba Linda, where sediment deposition and erosion related to the uplift of the northern hills obscures the paleochannel, which may have continued northward but is now eroded and buried. Old gravel fan deposits have been identified in the Jurupa Hills east of the Prado Dam, which have been detailed in previous studies, and this boulder train may be related to the former landscape that was present when this gravel fan was deposited. Certainly, the environment in which this stream flowed occurred prior to the Talbert Gravel interlude of the Santa Ana River based on cross-sectional evidence depicting boulder deposits overlain by the Talbert Gravel and the stream must have been much more competent to carry such large rocks.