Coarse-grained basin-fill deposits (foreground) pre-dating the Colorado River are capped by younger alluvial fan deposits (middle ground left on skyline) which are interbedded with through-flowing Colorado River deposits. Fortification Hill basalts in the background and skyline overlie similar basin-fill deposits. Mead Slope fault lies between Fortification Hill and alluvial fan deposits.
Online Resources from USGS Felter & others, 2013, Preliminary geologic map of Black Canyon and surrounding region, Nevada and Arizona. Map plate and text. https://pubs.er.usgs.gov/publication/ofr20131267
