  The surface detector array of the Pierre Auger Observatory provides
information about the longitudinal development of the muonic component of
extensive air showers. Using the timing information from the flash
analog-to-digital converter traces of surface detectors far from the shower
core, it is possible to reconstruct a muon production depth distribution. We
characterize the goodness of this reconstruction for zenith angles around 60
deg. and different energies of the primary particle. From these distributions
we define X(mu)max as the depth along the shower axis where the production of
muons reaches maximum. We explore the potentiality of X(mu)max as a useful
observable to infer the mass composition of ultrahigh-energy cosmic rays.
Likewise, we assess its ability to constrain hadronic interaction models.
