Where your water goes

Rain that falls on San Diego ends up in the Pacific Ocean

1. Your sub-basin

   San Diego Coastal-Baja California

2. then

   Pacific Ocean Seaboard

3. It arrives here

   Pacific Ocean

West, down some of the steepest drainage on the continent, and then straight into the Pacific. These are the salmon rivers: five species run up them from the sea to spawn and die, and the carcasses carry ocean nitrogen back into the forest.

Basin 180703 (USGS hydrologic unit)

Under your feet

San Diego sits on Quaternary sedimentary rock

About 3 million years old — from the ice ages and since — the most recent chapter.

All the kinds of rock, and how each one forms →

1. At the surface3 million years oldNeither way

   Sedimentary rock

   Quaternary sedimentary · Quaternary

   What it is

   The family that covers about three quarters of the land surface, in a thin skin over everything else. Some geological maps say only this much, which still tells you it will be layered and may hold fossils.

   How it wears

   Along its bedding planes, layer by layer, at a rate set by whatever the layers are made of — which is why sedimentary country weathers into steps.

   What it means above ground

   Layered country with flat-topped hills and stepped valley sides. What grows on it depends entirely on whether the beds are limey or sandy, which is the question worth asking next.

2. Below, layer 223 million years oldNeither way

   Sedimentary rock

   Cenozoic sedimentary rocks · Miocene

   What it is

   The family that covers about three quarters of the land surface, in a thin skin over everything else. Some geological maps say only this much, which still tells you it will be layered and may hold fossils.