Where your water goes

Rain that falls on Boston ends up in the Atlantic Ocean

1. Your sub-basin

   Massachusetts-Rhode Island Coastal.

2. then

   Atlantic Ocean Seaboard

3. It arrives here

   Atlantic Ocean

Everything that falls here works its way east and north to the Atlantic. It is the drainage that carried the fur trade inland and the log drives back out, and the one Atlantic salmon and American eel still use — the eel travelling the whole length of it to spawn in the Sargasso Sea.

Basin 010900 (USGS hydrologic unit)

Under your feet

Boston sits on Pennsylvanian sedimentary rock

About 323 million years old — from the age of the first forests, amphibians and reptiles.

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

1. At the surface323 million years oldNeither way

   Sedimentary rock

   Paleozoic sedimentary rocks · Pennsylvanian

   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 21.0 billion years oldNeither way

   Sedimentary rock

   Neoproterozoic-Cambrian sedimentary · Neoproterozoic

   What it is