Strata (singular: stratum) are the horizontal layers, or beds, present in most sedimentary rocks. During or immediately after the accumulation of sediments, physical, biological, and chemical processes produce sedimentary structures. Strata are probably the most common sedimentary structures, as almost all sedimentary rocks display some type of bedding. A rock that contains beds is stratified or displays stratification.
Strata form during sediment deposition, that is, the laying down of sediment. Meanwhile, if a change in current speed or sediment grain size occurs or perhaps the sediment supply is cut off, a bedding plane forms. Bedding planes are surfaces that separate one stratum from another. Bedding planes can also form when the upper part of a sediment layer is eroded away before the next episode of deposition. Strata separated by a bedding plane may have different grain sizes, grain compositions, or colors. Sometimes these other traits are better indicators of stratification as bedding planes may be very subtle.
The strata in an exposure or outcropping of sedimentary rock can range from layers as thin as paper, known as lamina (plural: laminae or laminations) to beds tens of feet thick. Generally, the more stable and consistent the environmental conditions during deposition, the thicker the strata. For example, in a river with very consistent current speeds, thick sediment layers with widely spaced bedding planes form. In a different river, where current speeds vary often, thin sediment layers and closely spaced bedding planes form instead.
The area covered by a bed, that is its areal extent, is also highly variable. Some beds can be traced for hundreds of square miles. Others may cover an area of only a few hundred square feet. Many factors influence the areal extent of beds. Among the more important factors is the setting in which the bed formed. Rivers, for obvious reasons, deposit beds shaped like a shoe string (long and thin); deposits in the open ocean often extend for great distances in all directions. Erosion of strata after deposition also affects their areal extent.
Bedding planes indicate variable environmental conditions during sediment deposition, but they may also be evidence of a gap in the geologic record. Many times a bedding plane develops because no sediment accumulates for at least a brief period of time or it is later eroded away. This represents an interval of time for which there is no sediment record. If we think of strata as a record of geologic time preserved in sediment (or sedimentary rock), this break or gap between sedimentation events actually is a gap in the geologic record. In other words, think of an outcrop of rock as being like a book; strata are pages and bedding planes are pages that have been torn from the book.
The time gap represented by a bedding plane may be very short, a fraction of a second, or perhaps a few minutes. After that interval passes, recording continues in the form of sediment deposition. However, sometimes the amount of time that is unrepresented (the gap) can be quite long, perhaps hundreds or thousands of years. These longer gaps are called unconformities. In some rock outcrops, more geologic time may be represented by the bedding planes (the gaps) than by the strata that lie between them.