Getting stoned refers to being under the influence of a drug (typically marijuana) and 1970s America experienced a strong war on drugs. A stone is a hard substance found in the ground, another word for it is rock. In ancient times, if someone did something criminal, they would get rocks thrown at them as punishment. This also means to get stoned. This joke is a play on the term "getting stoned".
The only similarity between ancient times and the 1970 ' s is that both were full of people getting stoned .