Edson and Colonel Gerald Thomas , Vandegrift 's operations officer , believed that the Japanese attack would come at the Lunga Ridge , a narrow , grassy , 1 @,@ 000 m ( 1 @,@ 100 yd ) long , coral ridge ( 9 ° 26 ′ 39 ″ S 160 ° 2 ′ 50 ″ E ) parallel to the Lunga River just south of Henderson Field . The ridge offered a natural avenue of approach to the airfield , commanded the surrounding area and was almost undefended . Edson and Thomas tried to persuade Vandegrift to move forces to defend the ridge , but Vandegrift refused , believing that the Japanese were more likely to attack along the coast . Finally , Thomas convinced Vandegrift that the ridge was a good location for Edson 's Raiders to " rest " from their actions of the preceding month . On 11 September , the 840 men of Edson 's unit — including the 1st Raiders and the Paramarines — deployed onto and around the ridge and prepared to defend it .