Neogene basaltic successions, one of the Early(?) and Middle Miocene age, the other of latest Miocene and Pliocene age, rest on land surfaces characterized by broad, low-relief components, above which rose hills that may never have been totally covered by the basaltic rocks. Local stream deposits, in places auriferous or uraniferous, have accumulated in channels cut into or running on the subbasalt surface and have been covered by the basalts. A subsequent, Late Pliocene and post-Pliocene differential uplift has produced elongate, northerly trending topographic highs and has led to major incision by streams. A third basaltic succession, which locally extends into the incised valleys, is recognized but is not discussed in detail.

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