Post-depositional remanent magnetization (PDRM) becomes permanent within a lock-in zone, whose apparent time delay (depth) and time span (thickness) are related to the characteristic time constants of compaction of magnetic grain rotation. Two simple models of the PDRM lock-in zone are described, using Yaskawa's idea of effective sedimentary viscosity, and Mooney's empirical relation for the viscosity of a concentrated suspension of spheres. The impulse response of the lock-in zone determines the fidelity of the paleomagnetic recording.