Generally these deposits are in the shape of flat sheets called strata, or beds. Remember that sediment settles out of water, air, or ice as loose particles. Gravity spreads this material out into horizontal sheets.
Layers of sediment are separated from one another by bedding planes, which mark brief interruptions in deposition. Bedding planes give piles of sediment their characteristic layered or stratified look.
As sediment accumulates, older strata are buried under younger ones. Geologists say that the layers are superimposed upon one another, with the oldest at the bottom of the pile and the youngest at the top.
Look at this exposure of weakly-consolidated alluvial sediment. We may know nothing about the actual age of these layers, in years-ago, but we can literally see the relative time order in which they accumulated. The arrows mark the bedding planes between the stacked layers of clastic sediment.