In the geologic past, the polarity of earth's magnetic field is

In the geologic past, the polarity of earth's magnetic field is reversed.

In fact, polarity of the earth's magnetic field changes periodically: position of magnetic north pole and magnetic south pole switch during the time, and this phenomenon is called geomagnetic reversal. The frequency of this reversal is not constant in time, and this phenomenon generally happens after thousands or milion years.