[en] Cross sections for the production of mass 6 to 11 isobars from proton spallation of carbon targets have been measured at bombarding energies of 45, 55, 60, 65, 75, and 100 MeV. Isotopic cross sections for Li and Be have also been measured at 100 MeV. The excitation functions for these reactions have been used to test theories of Li, Be, and B nucleosynthesis. The measured abundance ratios for ¹¹B/¹⁰B and B/Be can be reproduced using the experimental cross sections and astrophysical models which propose LiBeB synthesis in nuclear reactions well above the threshold energy. In order to obtain the natural ⁷Li/⁶Li, Li/Be, and Li/B abundance ratios, however, it is concluded that a substantial amount of ⁷Li must be synthesized via some lower energy mechanism