Determining the age period for the Acheulo-Yabrudian has been difficult as its major excavations occurred in the 1930s and 1950s before modern radiometric dating. The recently excavated Qesem and Tabun caves, however, suggest the oldest period is about 350 kyr and the most recent 200 kyr. This would make the Lower–Middle Palaeolithic transition rapid occurring at 215,000 BP within a 30,000 year period.[4] Some date it earlier at 400,000-220,000 bp.[5]