and December the Japanese emplaced field artillery on the high ground around the beachhead, concentrated in a group of hills along the Torokina River overlooking the eastern perimeter. They shelled the beachhead, targeting the airstrips and the supply dumps. The 3rd Marine Division extended its lines to include the hills in a series of operations that lasted from 9–27 December. One hill, dubbed "Hellzapoppin Ridge", was a natural fortress. Overlooking the beachhead, it was long, with steep slopes and a narrow crest. The Japanese constructed extensive positions on the reverse slopes using natural and artificial camouflage. The 21st Marines attacked Hellzapoppin Ridge but were driven off on 12 December. Several air strikes missed the narrow ridge completely. Finally, co-ordinated air, artillery, and infantry attacks resulted in the capture of the ridge on 18 December. In the days that followed, the 21st Marines were also involved in fighting around Hill 600A, which was captured by 24 December 1943. On 15 December, the I Marine Amphibious Corps and General Geiger were replaced by the US Army's XIV Corps, led by Major General Oscar W. Griswold, the victor of the land campaign on New Georgia. On 28 December, the 3rd Marine Division, exhausted because most of the fighting had taken place in its sector, was replaced by the Army's Americal Division under Major General John R. Hodge. The 37th Division (Army), was then placed under Griswold's XIV Corps. January–February 1944: encircling Rabaul Aerial reduction of Rabaul Rabaul had already been raided multiple times between 12 October and 2 November by the heavy bombers of General George C. Kenney's Allied Air Forces Southwest Pacific Area. Significant damage was done to ground installations, although the Japanese adapted by moving aircraft facilities underground. Only low-flying techniques such as dive bombing and glide bombing could achieve the accuracy required to pinpoint these installations, as well as neutralising anti-aircraft weapons and attacking vessels in the harbor. To achieve this, the Allies began constructing several airstrips on Bougainville that would allow them to use their smaller, more manoeuvrable, aircraft against Rabaul. The fighter strip on the beach at Torokina began operations on 10 December, while the inland bomber strip "Piva Uncle" followed on Christmas Day, and the inland fighter strip "Piva Yoke" on 22 January. General Ralph J. Mitchell, USMC, took over the command of all land-based planes in the theater, called Air Command, Solomons (Airsols), on 20 November.