Five laws of relative dating
Prior to the late 17th century, geologic time was thought to be the same as historical time.
Archbishop James Ussher of Armagh, Ireland, 1654, added up generations from the Old Testament and determined that Earth formed on October 23, 4004 BCE.
By carefully digging, we have found that each trash pit shows a sequence of layers.
Although the types of trash in each pit is quite variable, each layer has a distinctive kind of trash that distinguishes it from other layers in the pits.
Thus, we know that the dike is younger than the mudstone, sandstone, and shale.
How do we know this and how do we know the ages of other events in Earth history?Principle of Inclusions If we find a rock fragment enclosed within another rock, we say the fragment is an inclusion.If the enclosing rock is an igneous rock, the inclusions are called xenoliths.He concluded that process that are currently operating on the Earth must be the same processes that operated in the past.This principle is often stated as "the present is the key to the past".