This map of the Trinidad area is considered to be a special purpose map, as it shows only gravel-bearing alluvial deposits laid down by streams and a series of stacked lava flows erupted from local volcanoes.  Nongravelly surficial deposits and Cretaceous bedrock, which would be on a standard geologic map, are not shown.  The map has two purposes: a scientific one to show the distribution, character, and age of the alluvial deposits and a practical one, to map the distribution of deposits of gravel and crushable rock for use as construction materials.