Where your water goes

Rain that falls on Chicago ends up in the Gulf of Mexico

1. Your sub-basin

   Upper Illinois

2. then

   Mississippi River

3. then

   Mississippi River system

4. It arrives here

   Gulf of Mexico

This water joins the largest drainage in North America and travels south for thousands of kilometres to the Gulf. What runs off the fields upstream of you arrives there too, which is why a dead zone the size of a small country forms off Louisiana each summer.

Basin 071200 (USGS hydrologic unit)

Under your feet

Chicago sits on Paleozoic sedimentary rock

About 541 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 surface541 million years oldNeither way

   Sedimentary rock

   Paleozoic sedimentary rocks · Paleozoic

   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 2446 million years oldSweetens the soil

   Dolostone

   Silurian System undivided · Llandovery

   What it is

   Chemically limestone's sibling: calcium magnesium carbonate instead of calcium carbonate. The swap usually happened long after the original mud was laid down, as magnesium-rich water moved through it.