[Ask a Geologist] Before there was terraforming, there were rocks 1

My friend Andrew BartonĀ asked me, a bit out of the blue via twitter: Given a now-earthlike planet terraformed 15 million years ago, which previously resembled an Earth-sized Iapetus or something, how obvious would the pre-terraforming rocks be, or what governs how deep they’d be now? He also provided a bit more background as to the ...