Belt is dead, the dirt eroded the crap out of the groove sides to kill all grip. As well much dirt likely still stuck in belt carcass to make it slip even more. You ain't going anywhere without a new belt and then go from there, the tensioner may be dead too from dirt getting in it, they are not weather sealed. The pulleys with that much dirt may even have impacted it in the grooves to build the groove depth shallower and then all pulleys will need 100% of the concreted dirt chipped out before they will properly grab the belt again. You can look hard at the pulley and not see that and then the belt makes noise forever. The concreted dirt will appear to be part of pulley and requires extreme force to remove it. BTDT.