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- "He [Marty] saw something bright across a yard a few hundred feet in front of him. He walked toward it, and realized it was the kind of broad yellow tape cops used to keep crowds from walking all over the evidence: DO NOT CROSS — POLICE INVESTIGATION IN PROGRESS. Marty stopped just before the tape, and saw two chalk outlines on the sidewalk on the other side — outlines in the shape of people. Inside each outline were darker spots that glistened in the streetlights. Marty realized those spots must be blood."
- —From Back to the Future Part II by Craig Shaw Gardner (quote, page 103 and 104)
Chalk was a piece of soft white rock used for drawing and writing on a blackboard, as well as by the police to outline the bodies of victims in murder investigations.
The chalk outlines of two victims of a drive-by shooting, cordoned off by yellow POLICE LINE - DO NOT CROSS tape, were spotted in Lyon Estates by Marty McFly after he, Emmett Brown, Jennifer Parker and Einstein returned from 2015 in the DeLorean time machine to what they thought was 1985, but turned out to be a nightmarish alternate reality.
Later, Doc found a piece of chalk in his trashed lab, which he used to draw on a blackboard a diagram to explain to Marty about the ABC timeline where they had arrived.
