Abstract: A heavy duty, chassis-mounted rock trencher for use with a conventional tractor for digging a trench, which obviates the use of outboard counterweights typically used to prevent overcentering problems. The trencher comprises an elongated digger boom selectively pivotally deployable between an inclined digging position and a roughly horizontal transport position. Additionally, the boom may be selectively contracted or extended to transfer weight overcenter and enhance stability. The boom comprises a rigid steel arm entrained by a continuous ground-engaging track of conventional digger elements journalled for rotation about cooperating sprockets upon a headshaft driven by a conventional hydraulic motor. The boom is operationally secured within a rigid, hollow, generally rectangular boom control assembly comprising a movable headshaft mounting housing and a cooperating boom mounting carriage. When the boom is contracted, the headshaft housing slides within the carriage.