It is crap and blood.
Not like some unsuspecting plant eater step foot in La Brea. Cobblestones, concrete even asphalt are just thin veneer. Underneath is the sludge which makes the fabric of our lives.
Asphalt's been there from the very beginning. The kings which Abraham chased from Sodom and Gomorrah, stumbled and fell into the peat and tar of Asphaltitis, according to Josephus.
So watch your step. We stepped lightly on the sidewalks of New York City bypassing piles of dog dung, not realizing granite plates slid and panged on seas of refuse underneath. A 300 mph sht strm spinning out of control on its 23 degree imaginary axis.