ABSTRACT During the early Tertiary, in what is now the Sacramento Valley, four submarine canyons successively fed sediments into a deep remnant of the Mesozoic trough that formed the western margin of the North American continent. The Markley Valley is the youngest of these “canyons” having formed after deposition of the Sidney Shale Member of the Eocene Markley Formation. Cross sections show truncation of rocks as old as Cretaceous in the northern reaches of the valley but show that erosion of Tertiary rocks predominates in the southern end of the valley. The valley fill ranges from about 1.2 miles (2 km.) wide at its outcrop near Wheatland in Yuba County to greater than 12 miles (20 km.) wide west and southwest of Sacramento. In its thickest axial portion the valley fill is greater than 2000 feet (600 m.) thick. The Markley Valley is 67 miles (110 km.) long. The Markley Valley trends generally S10°W but north of Rio Vista it abruptly turns westerly to its presumed outlet. Structure contour maps of the base of the valley fill show the feature to be surprisingly irregular with reentrants that suggest tributaries and occasional highs which appear as monadnocks. Along much of its length the filling of the Markley Submarine Valley was the last marine event and the fill is overlain by continental sediments of the late Tertiary Tehama Formation. The Markley Valley Fill is dominated by shales and as such is less prospective for gas exploration than areas outside the valley. However, the Green’s Lake Gas Field 3 miles (5 km.) southwest of Sacramento gained a small portion of its production from what is locally called the Markley Valley Sandstone, clearly within valley filling sediments. This sand and others that are similar are thin and discontinuous compared to reservoir rocks outside of the valley fill. The Markley Valley more importantly impacts gas exploration as its fine-grained fill material truncates and seals thicker, more permeable reservoirs outside the valley. In as many as twelve gas fields including the Catlett, Conway Ranch, Fremont Landing, Karnak, Liberty Cut, Liberty Island, Maine Praire, Millar, Rio Jesus, Sacramento Airport, Todhunters Lake, and Winchester Lake fields, a significant amount of the gas trapped is attributable to valley-filling sediments truncating and sealing older gas reservoirs.