THE PILOT SAID HE WAS FLYING UP A VALLEY IN ROLLING HILL CONTRY AND ENCOUNTERED A SEVERE AND SUDDEN GUST OF WIND AS THE HELICOPTER FLEW OVER A RIDGE LINE AT A VERY LOW ALTITUDE. THE PILOT SAID THE WIND GUST WAS ACCCOMPANIED BY STRONG TURBULENCE AND A SEVERE DOWN DRAFT. THE PILOT SAID THE HELICOPTER WAS FORCED DOWNWARD INTO THE GROUND AND HE WAS ABLE TO ARREST THE DESCENT AND MAKE A NORMAL SOFT TOUCH DOWN ON THE CREST OF THE RIDGE. THE PILOT SAID THE HELICOPTER ROLLED OVER ON ITS SIDE JUST AFTER TOUCH DOWN.