The requirement is that you install the rod vertically 8' deep at an angle not greater than 45°. If this is not possible due to rock then you can bury the entire rod horizontally. I see no wording that would permit you to bend over and bury just the top portion of the rod.
250.53(G) Rod and Pipe Electrodes. The electrode shall be installed such that at least 2.44 m (8 ft) of length is in contact with the soil. It shall be driven to a depth of not less than 2.44 m (8 ft) except that, where rock bottom is encountered, the electrode shall be driven at an oblique angle not to exceed 45 degrees from the vertical or, where rock bottom is encountered at an angle up to 45 degrees, the electrode shall be permitted to be buried in a trench that is at least 750 mm (30 in.) deep. The upper end of the electrode shall be flush with or below ground level unless the aboveground end and the grounding electrode conductor attachment are protected against physical damage as specified in 250.10
All responses based on the 2011 NEC unless otherwise noted