Riverdale is built on a ridge, and every driveway up there has an opinion about gravity. Francis Property Solutions LLC works those grades in both directions of the calendar — mowing and edging the terraced lawns through the warm months, trimming the hedges that hold a hillside together, carting off the cleanups at either end of the growing season. Winter is the harder half: plowing and shoveling a sloped drive, then salting it twice, because a hill that looks clear at six can glaze over by eight. On a bright January morning the Hudson runs slate-colored below, the Palisades stand up on the far bank, and the driveway is down to bare pavement. That is the payment for the early alarm.