We just started a unit on rocks. My dad (a geologist) let me borrow a teacher's rock kit with ideas for teaching, and I also got some ideas from Core Knowledge and KONOS.
We've been talking about sedimentary rocks. Here's a definition from
A First Look at Rocks by Millicent Selsam and Joyce Hunt:
Everywhere on land the wind blows and the rain falls.
This makes rocks crack, break and crumble.
Bits and pieces of the rock tumble down into streams.
The streams carry them to the rivers
and the rivers carry them to the sea.
The bits and pieces can be small as sands grains
or as big as pebbles.
These are called sediments.
Millions of tons of these sediments pile up
layer upon layer.
The top layers press down on the lower layers.
Over millions of year, the sediment becomes sedimentary rock.
Yesterday we sorted though the rock kit and found the sedimentary rocks. Then we gathered some pebbles, sand, dirt, grass and soil and mixed them with water in a jar. It was a way for the kids to see how sedimentary rocks are formed. It's supposed to show their layers, but the kids couldn't really see them (too much grass and not enough pebbles).

Today we made some Seven Layer Magic Bars--another way to show the layers on a sedimentary rock. I must have made them wrong because I couldn't see the layers. At least it was a yummy mistake. Here's the
recipe and what they should have looked like:

Next up: Igneous rocks.