In the early 20th century geologists first noticed that some volcanic rocks were magnetized in a direction opposite to what was expected. The first examination of the timing of magnetic reversals was done by Motonori Matuyama in the 1920s, who observed that there were rocks in Japan whose magnetic fields were reversed and those were all of early Pleistocene age or older. At the time he published his proposal suggesting that the magnetic field had been reversed, the magnetic field itself was poorly understood so there was little interest in the possibility that it had reversed.