Geologic investigation of erosional surfaces east of Napa Valley, California

Remnants of at least two once-continuous erosional surfaces are preserved on six structural blocks in the study area. Similar volcanic sequences found on the structural blocks are separated by west-dipping normal faults related to the Soda Creek fault southeast of the study area. The single trace of the Soda Creek Fault splays into a more diffuse fault zone north of Rector Canyon.; Core stones found at the ground surface and in the soil are the spheroidally weathered remnants of the jointed bedrock. By using the thickness of the weathering rinds that form around the edges of core stones, it is possible to correlate the relative ages of the weathered surfaces. There is a distinguishable difference in rind thickness and, presumably, age between the erosional surfaces. When viewed graphically, gaps in the measured thickness of the weathering rinds may indicate periods of erosion corresponding to the uplift of the Vaca Mountains.