The park site, in the Ascot Hills, is a part of the Repetto Hills. The hills were formed from marine sedimentary deposits. Alternating layers of sands (from near shore deposition) and clays and silts (deposited in deeper water) were deposited as historic ocean levels rose and fell. Over time these sediments were compressed into the sedimentary bedrock formations. When the San Gabriel Mountains were uplifted by thrust faulting, these sedimentary formations were wrinkled up to form the low lying hills. In the Ascot Hills, the alternating layers have been tilted up along an anticline whose axis trends east- west, so the predominate bedrock slopes dip to south on the south side of this axis and to north on the north side of this axis. This folding and dip of these bedrock formations can be observed in some of the excavated banks along roads
The soils in the park all derived from the marine sedimentary bedrocks. There are no other naturally occurring rocks, such as granitic rocks that are usually transported from the mountains by erosion along the major washes and streams. Thus, the soils are either clays or silty clays derived from the weathering of siltstones or sands and sandy silts derived from sandstones. As one walks the park, traversing from south to north the change from clay soils to sandy soils, and then back to clay soils. This may be likened to walking across the edge of a layer cake that has been tilted up on its side. Local erosion features such as the stream in the valley and some colluvial deposition at the base of hills and in draws may add complexity to the distribution pattern of soils, but the primary variation is from the alternating bedrock layers of siltstones and sandstones.