  The standard tunneling model describes quite satisfactorily the thermal
properties of amorphous solids at temperatures $T<1K$ in terms of an ensemble
of two-level systems possessing logarithmically uniform distribution over their
tunneling amplitudes and uniform distribution over their asymmetry energies. In
particular, this distribution explains the observable logarithmic temperature
dependence of the dielectric constant. Yet, experiments have shown that at
ultralow temperatures $T<5mK$ such a temperature behavior breaks down and the
dielectric constant becomes temperature independent (plateau effect). In this
letter we suggest an explanation of this behavior exploiting the effect of the
nuclear quadrupole interaction on tunneling. We show that below a temperature
corresponding to the characteristic energy of the nuclear quadrupole
interaction the effective tunneling amplitude is reduced by a small overlap
factor of the nuclear quadrupole ground states in the left and right potential
wells of the tunneling system. It is just this reduction that explains the
plateau effect . We predict that the application of a sufficiently large
magnetic field $B>10T$ should restore the logarithmic dependence because of the
suppression of the nuclear quadrupole interaction.
