B REFLECT & DISCUSS The Vishnu Schist has a numerical age of about 1700 million years. The Lower Cambrian Tapeats Sandstone was deposited on top of the unconformity that you considered in part A. Imagine that you find reliable information indicating that the Tapeats Sandstone was deposited at the very start of the Cambrian Period and then how about much time is represented by the rock record that is "missing" across this unconformity—missing in the sense of erosion, nondeposition, or both prior to deposition of the Tapeats Sandstone on the erosional surface at the top of the Vishnu Schist?