We report results of off-lattice Monte Carlo simulations on randomly grafted polymer layers in poor solvent. The results of the investigations show that the conformations of polymer chains in these matrices undergo a gradual transition from a mushroom to brush shape with increases in grafting density, in agreement with expectations. We also find that the location of this transition can be predicted quantitatively with a knowledge of the isolated chain dimensions. Analysis of the detailed structure of the grafted layers supports recent theories which predict that clusters will form not only at low grafting densities, but also at higher densities in which the layer covers the entire surface. The segment density profiles evolve from a monotonic decay to the shape of a step function at moderate grafting densities, but show little agreement with analytical self-consistent field theories for brushes in poor solvent. Finally, we have examined brush structures at high grafting densities and find that the segments of these chains arrange into layers parallel to the surface due to packing constraints, but do not crystallize since the model employed is too simple to permit this possibility. Consequently, we find the formation of a kinetically controlled configurational glass phase, especially for long chain brushes.