Monday

Rocket drops a handful of gravel, then sand, then soil into a clear bottle of water and shakes it hard.

"Mud," he announces. "Just mud." He sets it on the Sky Deck step and reaches for the lid.

Raven holds up a hand. "Wait. What do you notice at the bottom?"

The gravel has already landed. Above it, sand is drifting down like slow snow. The water stays brown.

Nova hovers beside the bottle. "A geologist from long ago watched the same thing," she says. "He wrote a rule about it."

"Big stuff first," Rocket says. "Then smaller. Then the tiny bits, maybe never."

Raven writes it down. "Layers. Like pages." Nova hums. "And pages have an order."

The United States Geological Survey says we study Earth's history by studying the record preserved in rocks. The layers of the rocks are the pages in our history book.

Most of the rocks exposed at Earth's surface are sedimentary, formed from particles of older rocks broken apart by water or wind.

Gravel, sand and mud settle to the bottom in rivers, lakes and oceans. With time, more particles and chemical changes, the sediments at the bottom of the pile become rock.

Gravel becomes conglomerate. Sand becomes sandstone. Mud becomes mudstone or shale. Animal skeletons and plant pieces can become fossils.

In the mid-1600s the Danish scientist Nicholas Steno studied how sedimentary rocks sit. He found that solid particles settle from a fluid according to their weight or size.

The largest or heaviest settle first. The smallest or lightest settle last.

Slight changes in particle size or composition make layers, also called beds. Layering, or bedding, is the most obvious feature of sedimentary rocks.

Rock layers are also called strata. Stratigraphy is the science of strata, including how layered rocks relate to time.

Good start. Tomorrow you learn the three rules geologists use to put layers in order.