GR translation: the grotto of one eyeball

[Mike Jing]

V380.14-17, P386.17-20   Slothrop frees his arms from the cape, lets a lean
gray Porsche whir by, then charges out, the red of its taillights flashing
along his downstream leg, headlights of a fast-coming Army truck now
hitting the upstream one and touching the grotto of one eyeball to blue
jigsaw.

What does "the grotto of one eyeball" refer to exactly? And how does it
become a "blue jigsaw"?