About this map

Most of Los Angeles County is dominated by alluvium (44%), which occurs in the lowlands area in the Los Angeles Basin and Mojave Basin.  Sedimentary sandstone (19%), igneous grandodiorite (14%), and metamorphic gneiss (7%) also cover a significant area of Los Angeles.

  • Igneous rock: formed by cooling of magma under the earth’s surface (e.g. granite) or lava as it breaks the earth surface (e.g. volcanoes)
  • Sedimentary rock: cemented particles (e.g. clay, silt, sand, biological materials) that were formed by erosion and deposited on the floor of shallow oceans or lakes (e.g. sandstone)
  • Metamorphic rock: igneous or sedimentary rocks that have been transformed by heat and pressure (e.g. limestone to marble, silt to slate), generally deep within the earth
  • Alluvium isn’t a rock type, but is made up of deposits of other eroding materials (e.g. gravel, sand, silt, or clay) that were moved by water from higher to lower ground, where they form alluvial fans or terraces; these relatively young formations are often less than a million years old

The geology and geologic substrate influence biodiversity in Los Angeles in several ways. First, the region’s varied geology creates diverse topographic, climate, and vegetation gradients, which plants and animals have evolved and adapted to over millions of years. Second, certain species of plants have evolved to depend on unique geologic rock types such as volcanic rock, serpentine, limestone, or unique substrates such as vernal pools or washes.  Third, alluvium tends to occur in relatively flat, low-lying areas that are ideal for development. Thus, the native vegetation (e.g. wildflowers) and ecosystems (e.g. vernal pools, oak woodlands, wetlands, and riparian areas) that occurred in these areas of the Los Angeles Basin are now very rare.

Data source: USGS Open-File Report (2005-1305): Preliminary integrated geologic map databases for the United States.