We present the results of simulations and theoretical investigations of a system of particles with a weak, long-range repulsive interaction. At fixed density the fluid phase, which is stable at high temperatures, becomes metastable and then unstable as the temperature is lowered. For low temperatures the particles form approximately equal size clumps that interact weakly with neighboring clumps. The global free energy minimum at low temperatures corresponds to a crystalline lattice clump structure. However the free energy surface has many minima associated with metastable amorphous phases. Quenches from the fluid phase to low temperatures almost always will result in an amorphous clump structure with the number of clumps dependent on quench history. The association of the amorphous phase with a large number of metastable minima is similar to a mean-field spin glass.