Imaged by David Harrington.
The distant triangular shadow is that of Haleakala, Hawaii where David was standing during sunset. The shadow is not cast on the clouds but instead is a long three dimensional duct of unlit air tapering away from the mountain.
Mountain shadows appear triangular more or less independent of the mountain profile. What you see are the edges of the dark duct. Look carefully for anti-crepuscular rays.
Image ©David Harrington, shown with permission.