Sally saw it first. The crack ran across three sidewalk panels. Dale stopped and looked at it. It started at the edge of the first panel, crossed the seam, continued through the middle of the second, and ended somewhere under the base of the oak tree at the curb. The middle panel had heaved up on one side, maybe half an inch, maybe a little more. You could see it if you looked. Sally looked at it. "It's not damaged," Dale said. "It's migrating." Sally looked at the raised edge of the panel. "Concrete migrates toward pressure relief," Dale said. "When the substrate shifts, the surface finds a new position. That's what you're seeing." "The substrate," Sally said. "What's underneath," Dale said. "Soil composition. Root behavior. The whole column." He crouched down and looked at where the crack met the tree base. The root had pushed up under the panel edge, lifting it cleanly. "The root is load-bearing," Dale said. "It's actually supporting the panel. If you removed the root the panel would drop." Sally looked at the root. It was clearly pushing the panel up. "That's why the city leaves them," Dale said. "People think it's neglect. It's actually maintenance."

Sally looked up the street. There were four more cracked panels between here and the corner, each one lifted slightly at the tree line. She didn't say anything about them. Dale stood and walked to the raised edge of the middle panel. He looked at the half-inch lift. Then he stepped up onto it, onto the high side, to test the stability. The panel rocked slightly under his weight and his ankle turned and he stepped off it fast. He looked at the panel. "Still finding position," he said. Sally looked at his ankle. He was putting weight on it normally. It was fine. "How long does that take," Sally said. "Depends on the root system," Dale said. "Active roots move slow. You wouldn't notice day to day. Over a season you'd see the change." "It moved just now," Sally said. Dale looked at the panel. "That's surface adjustment," he said. He looked at it a moment longer. "The panel is settling into the load. That's different from migration."

Sally looked at the crack where it crossed the seam between panels. There was a dark line of old dirt in it, and below that newer pale concrete showing where the crack had widened recently. Dale crouched again and looked at the crack line. "Consistent with gradual migration," he said. He looked at it a moment longer. "No stress fracturing. The propagation is clean." Sally crouched next to him. She looked at the pale new concrete inside the crack. She put one finger into it without touching the sides and felt how deep it went. She didn't say how deep it went. They both stayed crouched for a moment looking at the crack. "Good crack," Dale said. He stood and walked on.

Sally stayed crouched. She looked at the pale new concrete inside the crack and the dark old dirt above it and the root running under the panel edge and the three other panels visible from here, each one lifted slightly at the tree line, each one with its own crack running in its own direction toward its own root. Then she followed him.