A flightless rock auger is used for drilling postholes through rock with the use of hydraulic pressure and at slow revolution per minute. The rock auger is used in combination with a conventional flighted auger used for drilling postholes in dirt and clay. During a posthole drilling operation, the rock auger is substituted for a conventional flighted auger as needed for drilling through and removing rock or other hard material such as concrete from the posthole. The rock auger is designed for use at very low revolutions per minute and can function utilizing only the weight of the auger; however, the rock auger is designed for use with pressure drilling units as well for cutting a circular hole through the rock and forming a plug which is to be lodged in the cavity of the rock auger cylinder to be removed from the posthole. The quick connection enables the shaft of a pressure drill unit to engage the flightless auger to be quickly substituted for the rock auger to facilitate fast removal of soft dirt from the posthole. A pilot drill bit can be disposed within the cutting head for extending outwardly pass the cutting edge thereof for starting the hole, breaking up rock, and holding the auger in position during the drilling operation. A portion of the cylindrical body can be removed to allow quick clearance of the plug.