1. Where two units are in depositional contact, the older unit will still be present in the subsurface beneath the younger one. A drillhole at X will pass first through Cretaceous (K) then through Jurassic (J) and finally through Triassic (Tr)
2. Where an old unit exists at the surface and younger units are nearby, it is safe to assume the younger units once overlay the older rocks but have been stripped away by erosion. If you drilled at X in the diagram, you would not hit Jurassic or Cretaceous rocks at all, because they would originally have been above X.
3-4. Where an old unit disappears beneath a younger one, say at an unconformity, the young units merely conceal the older units. If you could strip away the young unit, you would see a continuation of the outcrop patterns that are present elsewhere on the map.
May 4th, 2015
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