Geologic map and schematic geologic cross-section of Kauai. Such maps display the distribution, orientation and mutual relationships of major rock types. For a geologist they are absolutely critical displays. This map, published in 1960, still displays the main geologic features of Kauai. Island exposures of the Napali Member of the Waimea Canyon Basalt (shown in red) have been dated radiometrically at 4.35-5.10 million years old; however, those rocks represent only the uppermost portion of the basaltic rocks that formed the shield volcanic mountain-island. Note how the lava flows dip gently, at 8-10 away from the center of the island. Similar volcanic rocks comprise the bulk of the mountain to its base, some 18,000 below sea level. Hawaiian volcanoes are built up from the sea floor in as little as 0.5 to 1.5 million years, so the oldest portion of Kauai would only be around 6 million years old. The Olokele Member (orange) is composed of somewhat thicker, nearly flat-lying basalt flows thought to represent post-shield, caldera-filling volcanics. Only one radiometric date, at 3.95 my, has ever been reported for those lava flows. The Makaweli Member (green), at 4.16-3.92 my, fills a down-dropped fault-like feature known as the Makaweli Graben. The Haupu Member (brown) apparently represents lavas that filled a minor flank caldera near the southeastern portion of the present island. Koloa Volcanics (blue) represent the rejuvenation stage lava flows and explosive vent deposits that erupted around 3.65-0.52 million years ago. Although they cover most of the eastern portion of the island, they represent only a thin, veneer-like volcanic layer that comprises less than 1% of the bulk of the mountain-island. Sedimentary deposits represented by the Palikea Formation (purple) occur mainly between the Waimea Canyon Basalt and the Koloa Volcanics and represent mainly cliff talus and stream sediments eroded and deposited during the brief break in volcanic activity between the shield-building and rejuvenation stages. Finally, note the relatively young sedimentary deposits (yellow) of the various river valleys and the Mana Coastal Plain. (Map and cross-section modified from Macdonald, Davis and Cox, 1960).