For several million years in the late Miocene, the Ridge Basin of southern California (33°42’40.00” N, 116°14’40.95” W) alternated between lacustrine facies (Peace Valley Formation) and terrestrial facies (Ridge Route Formation). The Alamos Canyon Siltstone, ∼1800 m in thickness, was deposited ∼5.5 Ma as the last lacustrine member of the Peace Valley Formation. It is primarily distinguished from the underlying Posey Canyon Shale by finer grain size. A coarse-resolution study was conducted to determine how the lake evolved during this last depositional phase. Most of the previously described outcrops of the Alamos Canyon Siltstone (ACM) are submerged beneath Pyramid Reservoir and thus sampling focused on five discrete subsections that had adequate exposure. The bottom (1835 to 1815 m), middle (732 to 481 m), and top (0 to 63 m) exposures are mostly silt, indicating deposition in calm waters. Two cross-bedded sandstone sections (487 to 481 m and 1096 to 1069 m depth) with associated turbidite and debris flows clearly indicate higher energy deposition and/or the progradation of streams during lower lake levels. Minerology and stable-isotope geochemistry of the lacustrine facies suggest that the lake was anoxic and brackish during the entire depositional history of the ACM. Dolomite is the primary carbonate in the top and bottom subsections. The δ¹⁸Odol values are positive for both units but increase towards the top, suggesting related evaporative concentration of δ¹⁸O due to the possible closing of the basin. In contrast, a marked shift from positive to negative δ ¹³C_δol values toward the top implies a shift in the mode of formation with microbial activity playing a more significant role in the breakdown of organic matter as expected under anoxic conditions.