The Crevasse is a deep crack or fracture in the Land of the Lost forming a natural boundary between the hunting grounds of its top two predators, namely, Grumpy, who roams the boundary of the swamp and marshlands, and Big Alice, who controls the land around the Lost City. It appears to be several miles long although this can't be confirmed as the attributes of the Land of the Lost affect laws of distance (its boundaries are linked so that one entering one valley just re-enters at the opposing cardinal point), and it's distance ranges from fifteen to twenty feet wide at its narrowest to fifty feet at its widest. The Marshalls have made much use of a huge stone block resembling a land bridge in their trips to and from the Lost City.
Not much is known about how deep it is or how it was formed. It seems to be in excess of a hundred feet deep, but this hasn't been confirmed. Will Marshall, Holly Marshall and Cha-Ka have fallen into it, landing on a small ledge some twelve to fifteen feet below the top, and getting rescued by Rick Marshall. It seems to have been cut by the river flowing out of the Land of the Lost and to the waterfall near the caves of Jefferson Davis Collie III, but it is unrevealed if this is the same crevasse separating Grumpy's hunting grounds (or Alice's hunting grounds) from the lands of the Tappa. It is known that since the devastation to the area that it may have narrowed or been pressed into non-existence since Grumpy and Big Alice now have more direct access to each other afterward.
It is known that through the observations of Rick Marshall that the river in the Land of the Lost makes a complete loop. If this is the case, just what keeps the river flowing back uphill from the waterfall to rejoin its headlands is unexplained.