Free 5-day trial Discover how geologists study the layers in sedimentary rock to establish relative age.
Learn how inclusions and unconformities can tell us stories about the geologic past.
From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone.
Photo from Glen Canyon National Recreation Area, Utah.
Today we apply this principle across the Grand Canyon—even across oceans to link continents that once were adjoined. in the plane of the [crystal] axis both the number and the length of the sides are changed in various ways without changing the angles."The other principles are often called Steno's Laws, but this one stands alone at the foundation of crystallography.
"If a body or discontinuity cuts across a stratum, it must have formed after that stratum."This principle is essential in studying all kinds of rocks, not just sedimentary ones. It explains just what it is about mineral crystals that make them distinct and identifiable even when their overall shapes may differ—the angles between their faces.
In 1669, Niels Stensen (1638-1686), better known then and now by his Latinized name Nicolaus Steno, formulated a few basic rules that helped him make sense of the rocks of Tuscany and the various objects contained within them.
His short preliminary work, (Provisional report on solid bodies naturally embedded in other solids), included several propositions that have since become fundamental to geologists studying all kinds of rocks.The Law of Superposition, which states that older layers will be deeper in a site than more recent layers, was the summary outcome of 'relative dating' as observed in geology from the 17th century to the early 20th century.The regular order of occurrence of fossils in rock layers was discovered around 1800 by William Smith.Geologists establish the age of rocks in two ways: numerical dating and relative dating.Numerical dating determines the actual ages of rocks through the study of radioactive decay.Relative dating uses the principles or laws of stratigraphy to order sequences of rock strata.