Pictured above is a little piece of geologic history known as a basal conglomerate. that’s a rock formed after a period of erosion that marks the boundary between two geologic time periods: in this case, the Mississippian (359-318 million years ago) and the Pennsylvanian (318-299 million years ago).
318 million years ago sea level subsided, bedrock was exposed, and the Mississippian Period came to an end. When exposed to erosion at the earth’s surface, pieces break off from bedrock. Flowing water in rivers, streams and oceans wears the edges of those rock fragments till they’re rounded. Once ocean level rises and deposition resumes, the rounded gravel gets mixed with newly accumulating sediment and forms a rock which is made partly of fragments of the older bedrock. Geologists call this type of rock a basal (at the base) conglomerate (containing round gravel) because it is the first bedrock signaling the beginning of a new period of geologic time.