A variety of seismic experiments indicate that seismic velocities in the uppermost oceanic crust (layer 2A) increase very rapidly with age, nearly doubling in 10 my or less, but only one measurement was made in situ, by an oblique seismic experiment in DSDP/ODP Hole 504B. A VSP acquired in 11 Ma crust at ODP Site 1243 yields a velocity of 4.21 ¿ 0.08 km s-1, which is consistent with previous observations. The absence of a vertical velocity gradient at Site 1243 is consistent with the hypothesis that layer 2A is modified by a process of progressive sealing of cracks from the bottom up. Application of an asperity deformation model suggests that the area of contact across cracks in layer 2A increases by a factor of ~100 between 1 and 10 my.