The Tusker Rock is a plateau with a rocky pinnacle above water, and a wall which drops off some 70m to the north. When the tide is running northwards in Jack Sound, it splits round Tusker Rock, swirls into back eddies to the north of the rock, smoothes across the plateau, then tumbles into vicious down- and up-currents as it passes across the wall into deeper water. Admiralty Sailing Directions for 1870 noted the strong eddy and the soundings which dropped from 6-7 fathoms to 21 fathoms off the wall.

Sources include:
Admiralty, 1870, Sailing Directions for the West Coast of England from Milford Haven to the Mull of Galloway including the Isle of Man, pg2-3

Maritime Officer, RCAHMW, August 2014.