The age relationship between sandstone and limestone is a common geological inquiry. Both sedimentary rock types are found globally, preserving vast stretches of Earth’s history. Determining which rock formed first depends entirely on the specific geological history of the location being examined, requiring an understanding of their origins and the principles used to read the rock record.
The Fundamental Difference: Formation and Composition
Sandstone and limestone are distinct because they form from different materials and in vastly different environments. Sandstone is classified as a clastic sedimentary rock, meaning it is made up of fragments of pre-existing rock and mineral material. These fragments are typically sand-sized grains, primarily composed of durable minerals like quartz and feldspar, which are cemented together over time.
The formation of sandstone results from the physical weathering and erosion of continental landmasses, with the sand being transported and deposited in high-energy settings. Common depositional environments include riverbeds, deltas, deserts, and shallow-marine beach areas where water or wind currents are strong enough to move the coarse sand grains.
Limestone, in contrast, is primarily a chemical or biogenic sedimentary rock composed mainly of the mineral calcite, which is a form of calcium carbonate. This rock forms in low-energy marine environments, most often far from the influx of continental sediments that create sandstone.
The vast majority of limestone is created through the accumulation of skeletal fragments and shells from marine organisms, such as corals and mollusks, or through the chemical precipitation of calcium carbonate from seawater. This biological origin means limestone is a record of ancient marine life and requires clear, warm, shallow to deep ocean water to form effectively.
Determining Relative Age Using Stratigraphy
Geologists determine the relative age of rock layers using stratigraphy, the study of rock strata. This process establishes a chronological sequence without determining an exact numerical age. The fundamental concept applied to layered sedimentary rocks is the Law of Superposition, formalized in the 17th century.
The Law of Superposition states that in any undisturbed sequence of rock layers, the layer found at the bottom is the oldest, and the layers found above it are progressively younger. This principle is intuitive; a layer of sediment must be present first before a new layer can be deposited on top of it.
Reading the vertical arrangement of rock layers, or the stratigraphic column, establishes the order of events at that location. If sandstone is found beneath limestone, the sandstone is older. Conversely, if limestone is situated beneath sandstone, the limestone is the older rock layer.
These principles of relative dating are applied globally to correlate rock units and reconstruct Earth’s geological history. The vertical stacking order of the sandstone and limestone layers is a direct record of the sequence of depositional environments that existed over time.
Typical Stacking Order of Sandstone and Limestone
The specific stacking order is controlled by changes in global or local sea level, which causes depositional environments to shift laterally. When sea level rises, a marine transgression occurs, and the shoreline moves inland. During a transgression, the environments that create the rock types shift landward and begin to stack.
In this common scenario, the shallow-water sandstone environment is deposited first, followed by the deeper-water limestone environment as the sea encroaches. The resulting sequence, from bottom (oldest) to top (youngest), is sandstone beneath limestone. This means the sandstone is the older rock, recording the deepening of the water over that area.
The opposite scenario, a marine regression, occurs when sea level falls and the shoreline moves seaward. The shallow-water environments then prograde, or advance over, the deeper-water deposits that formed previously.
A regressive sequence results in the deeper-water limestone being deposited first, followed by the shallower-water sandstone. In this case, the limestone is the older rock, found beneath the sandstone. Geologists use the grain size and composition of the rocks to interpret which sequence of environmental change occurred, determining which rock is older at that location.