Geomagnetic dating of rocks is possible because
This process, in which the rotation of a planet with an iron core produces a magnetic field, is called a dynamo effect.
The Earth's magnetic core is generally inclined at an 11 degree angle from the Earth's axis of rotation.
However, each time the firepit is reheated above the Curie point while being used to cook something, or provide heat, the magnetization is reset.
Therefore, you would use archaeomagnetic dating to date the last time the firepit was heated above the Curie point temperature.
They make a marking on the top of the core which indicates the location of the magnetic north pole at the time the core was collected. Paleomagnetism: Magnetic Domains to Geologic Terrains. When lava and clay are heated, or lake and ocean sediments settle through the water, they acquire a magnetization parallel to the Earth's magnetic field.After they cool or settle, they maintain this magnetization, unless they are reheated or disturbed.Archaeologists assemble a large number of these ancient VGPs and construct a composite curve of polar wandering (a VGP curve).The VGP curve can then be used as a master record, against which the VGPs of samples of unknown age can be compared to and assigned a date.