The earth may have reversed the direction of its field several times during its history of 4 to 5 billion years. How can geologists know about the earth field in such distant past ?

Question

$\begin{array}{1 1} \text{Analytical study of rocks } \\ \text{Analysis about the earth's core} \\ \text{Analysis about the earth's crust } \\ \text{ Analysis about magnetic field surrounding the earth } \end{array} $