Q: Passage: The "basket", an angling chain-link fence that runs along the top of the outfield wall, was installed at the start of the 1970 season. During the 1969 pennant race, there were several incidents of fans interfering with fly balls and even falling onto the field. There was also the first and only incident of a fan running the field and escaping without prosecution. A famous photograph was taken of the incident and published in the Chicago Sun-Times. The basket was intended to deter or prevent that kind of problem. Security cameras were also installed at that time. The "basket" angles away from the wall, and is also higher than the wall in order to provide some balance for the pitchers. However, over the years a number of baseballs have been hit "into the basket" for home runs that previously would have been outs, or off the wall, or possibly interfered with by fans. The basket only exists where there is seating. During the 1980s, when the bleacher seating was extended over the "catwalks", i.e. the bleacher ramps in extreme left and right fields, the basket was likewise extended.

Question: Based on this passage, when was the basket installed at wrigley field? Let's give stream of consciousness first:
To answer the above question, the relevant sentence is: The "basket", an angling chain-link fence that runs along the top of the outfield wall, was installed at the start of the 1970 season. Therefore, the final answer is the start of the 1970 season.