  Stellar evolution theories predict a gap in the black hole birth mass
spectrum as the result of pair instability processes in the cores of massive
stars. This gap, however, is not seen in the binary black hole masses inferred
from gravitational wave data. One explanation is that black holes form
dynamically in dense star clusters where smaller black holes merge to form more
massive black holes, populating the mass gap. We show that this model predicts
a distribution of the effective and precessing spin parameters, $\chi_{\rm
eff}$ and $\chi_{\rm p}$, within the mass gap that is insensitive to
assumptions about black hole natal spins. We analyze the distribution of
$\chi_{\rm eff}$ as a function of primary mass for the black hole binaries in
the third gravitational wave transient catalog. We infer the presence of a
high-mass population of black holes that is consistent with hierarchical
formation in dense star clusters. This population becomes dominant above
$44^{+6}_{-4} M_\odot$, which we interpret as the lower edge of the
pair-instability mass gap. Upcoming data will enable us to tightly constrain
the hierarchical formation hypothesis and refine our understanding of binary
black hole formation.
