Reading the Rock Record
Every sedimentary layer tells a story of the environment where it formed — a beach, a river, a deep ocean floor. Johannes Walther recognized in 1894 that the vertical succession of layers in a cliff or drill core directly reflects the lateral arrangement of environments at the time of deposition. This deceptively simple insight, known as Walther's law, is the key to reconstructing ancient geography from sedimentary rocks.
Accommodation and Sediment Supply
The architecture of sedimentary successions is controlled by the balance between accommodation space (room for sediment, created by subsidence and sea-level rise) and sediment supply. When accommodation exceeds supply, the shoreline retreats landward and deeper-water facies stack upward (retrogradation). When supply exceeds accommodation, the shoreline advances seaward and shallow-water facies build outward (progradation). This A/S ratio is the master control on stratigraphy.
Sequence Stratigraphy
Sequence stratigraphy, developed in the 1970s-1980s, organizes the rock record into depositional sequences bounded by unconformities. Each sequence contains systems tracts — lowstand, transgressive, highstand, and falling-stage — that reflect different phases of relative sea-level change. This framework revolutionized petroleum exploration by predicting the distribution of reservoir sands, source rocks, and seals within sedimentary basins.
Building a Stratigraphic Column
This simulation lets you adjust sea-level change, sediment supply, and subsidence to build a synthetic stratigraphic column and observe how facies stack vertically. Watch the shoreline migrate, see unconformities form during sea-level falls, and develop intuition for the relationships between accommodation, supply, and the resulting sedimentary architecture that subsurface geologists interpret from well logs and seismic data.