Rock fragments also occur in the composition of siliciclastic sedimentary rocks and are responsible for about 10 - 15 percent of the composition of sandstone.They generally make up most of the gravel size particles in conglomerates but contribute only a very small amount to the composition of mudrocks.ock fragments also occur in the composition of siliciclastic sedimentary rocks and are responsible for about 10 - 15 percent of the composition of sandstone.