Flat-floored, quasi-circular depressions on the southern residual cap of Mars have been observed to expand rapidly. Analysis of the size distribution combined with modeling of the growth process yields information about the ages of these features. We report on properties of a population of these features within a selected study area. We find a narrow size distribution that indicates a common formation time and a finite period during which new features were forming. Some change in environmental conditions occurred during this period, which we date at several Martian decades to centuries ago depending on modeled growth rates. We report on THEMIS data that reveals the nature of larger features outside our study area. Underlying water ice is exposed on the flat floors of these depressions near the outer walls. These features may record more than one growth phase, indicating that conditions conducive to growth may switch on and off with characteristic timescales of Martian centuries.