9.
The line, to the left of the post, extending through the rock on both sides of the mesa, is one of the principal causes of the development of the box canyon. After El Morro was buried by several thousand feet of younger rocks, some 60 million years ago, it was subjected to great pressures from the weight of the overlying rocks and the movements which caused the Zuñi Mountains to project so far above sea level. These pressures caused the sandstone to crack into the long openings which we call joints. As running water and wind gradually removed the overlying rocks, the waters were able to run down into the joints and, alternately freezing and thawing, broke up the rock into small fragments which could be washed away.
If you stand on this line and look down the box canyon to the west, you can see that each steep canyon wall is simply one side of a joint, while the material on the other side has been eroded away. If you look eastward towards the headquarters area, along this joint line, you can see how El Morro maintains its vertical walls by breaking into blocks which fall away from the main mass of the sandstone and leave a vertical joint face. This joint line is different from the others which you may have noticed in climbing over the rock because it goes so far and cuts through the rock so deeply.