The idea of the principle of superposition in stratigraphy is that the layer on the bottom in a stratigraphic sequence is the youngest layer. the layer on the bottom in a stratigraphic sequence is the oldest layer. the decay at a known rate of the radioactive isotope in the stratigraphy makes it possible to determine the date. all layers in a stratigraphic sequence date to the same time. An artifact is a. an object made by humans, such as a stone tool. a person who excavates and studies artifacts. A location with evidence of past human activity, including artifacts and other remains. an object used by humans, but not modified by them, such as plant remains. An event that occurred at the end of the Mesolithic was the first appearance of life on earth. first appearance of reptiles. first appearance of the therapsids. extinction of the dinosaurs. Solution 31.b.The principle of superposition states that rock forms horizontally with the oldest layers of rock lying at the bottom of a formation and newer rock resting on top. 32.a.Any object made by human beings,especially with a view to subsequent use.It is a handmade object. 33.4.extinction of dinosaurs.