Near the bottom of the Grand Canyon, we can see the boundary between the sedimentary veneer, or cover (here, a succession of horizontal layers), and the older basement (here, the steep cliff of dark metamorphic rock that goes down to the river). A geologist’s sketch emphasizes the contact, or boundary, between cover and basement.

In this photo of the Grand Canyon, we can see five formations. Formations and groups are examples of stratigraphic units. Note that each formation consists of many beds, and that beds range greatly in thickness. The boundaries between units are called “contacts.”

When blowing sand builds into sand dunes in a desert, the sand tumbles up the windward side of the dune, and settles in quieter air on the leeward side. This animation shows how cross beds develop during the deposition of sediment.

On this cliff face of sandstone in Zion National Park, we see remnants of ancient sand dunes. Cross beds indicate the wind direction during deposition.

This exposure shows the lens-like shape of an ancient gravel-filled river channel in cross section. A geologist’s sketch emphasizes the channel shape.

As sea level rises the coast migrates inland (transgression) and retreats seaward (regression), and a record of this movement is preserved in the strata of the sedimentary basin. View 1 shows how this sedimentary sequence is formed; View 2 examines a segment of the landscape millions of years later, after the land has been uplifted and erosion has occurred.