  We report the observation of a steepening in the cosmic ray energy spectrum
of heavy primary particles at about $8 \times 10^{16}$ eV. This structure is
also seen in the all-particle energy spectrum, but is less significant. Whereas
the `knee' of the cosmic ray spectrum at $3-5 \times 10^{15}$ eV was assigned
to light primary masses by the KASCADE experiment, the new structure found by
the KASCADE-Grande experiment is caused by heavy primaries. The result is
obtained by independent measurements of the charged particle and muon
components of the secondary particles of extensive air showers in the primary
energy range of $10^{16}$ to $10^{18}$ eV. The data are analyzed on a
single-event basis taking into account also the correlation of the two
observables.
