At 2:40 p.m. fire/EMS units were dispatched for a "RESCUE INACCESSIBLE TERRAIN" incident on the trails near the railroad right of way on Rosewick Road in La Plata. Information received at the 911 center revealed a male subject riding on the rear of a ATV fell off, striking his head, unconscious with "a hole on the back of his head". Caller also reported attempts were being made to control the bleeding. Chief 1A (Gilroy) arrived "establishing command" with RE13 and PA519 and instructed Utility #1 to take paramedics to the scene to evaluate the inquired person. Chief 1 (Sekuterski) advised command patient is a Priority #1 Cat A, MSP Trooper 2 was pre-launched and landing at FOP lodge 1/4 mile from scene. Utility #1 transported patient to LZ with paramedics on-board. RE123 handled the LZ, Trooper 2 transported to Trauma Center. Command terminated, CCSO continuing this investigation.