When the pathway narrowed it became more of a canyon featuring the oldest rock at Joshua Tree: pinto gneiss.
http://digital-desert.com/joshua-tree-n ... ad/14.html The banded and folded Pinto gneiss, approximately 1.7 billion years old, is most likely the oldest type of rock in the park. Gneiss, a metamorphic rock, has undergone change in mineral composition, grain size and orientation due to increases in pressure, heat and chemical activity. Gneiss, prior to its change, was a sedimentary rock. With directed pressure, mineral grains segregate and band together; this alternate banding of light and dark minerals is what defines a gneiss.