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That keyhole passage; keyhole is 1. this very descriptive name derives from the crosssectional shape of a cave passage that consists of a phreatic tube with a vadose canyon cut in its floor. it is the classic example of a two-phase cave passage that originated and began its development in the phreas and was then modified by vadose entrenchment. as this sequence is the result of water table lowering by normal surface erosion, keyholes are common. some keyholes are so small that the lower slot is impassable and the caver has to squeeze along the upper tube; others are very large. spectacularly long is the 5km of keyhole forming the fissures in castleguard cave, canada. a tube 6m in diameter tops an irregular tapering canyon 15m deep that must be traversed on sloping ledges at mid-level [9]. 2. a small passage or opening in a cave; in cross section, rounded at the top, constricted in the middle, and rectangular or flared out below [10]. they appear as keyholes when viewed in cross section. they are formed when underground streams flowing in a tubular passage begin downcutting to form a canyon passage [15]. see also canyon passage; passage; tubular passage; vertical shaft.?

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