Looking north you are viewing several volcanic cinder pits which were probably used in the early construction of Rout 66. The early road builders were limited by not only the equipment that was available but also by the building materials they could utilize.
The San Francisco Volcanic field which stretches from near Williams to Leupp provided some of these materials in the form of volcanic cinders. An advantage of cinders as a road construction material is the relative ease with which they can be dug from the quarry, a mechanical shovel or loader were usually adequate for their extraction although occasionally a bulldozer was required to open up a working face. The early use of shovels resulted in the vertical face we see on many of the old pits.
Cinders were used in all stages of early Route 66, in the sub-base they were generally hauled straight from the source to the road bed and compacted as is. For the base material they would be screened to remove any large material and placed on top of the sub-base. In some cases cinders were even used in the final surface mixed with asphalt or used in macadam.
Wikipedia says McAdam's method was simpler, yet more effective at protecting roadways: he discovered that massive foundations of rock upon rock were unnecessary, and asserted that native soil alone would support the road and traffic upon it, as long as it was covered by a road crust that would protect the soil underneath from water and wear. With the advent of motor vehicles, dust became a serious problem on macadam roads. The area of low air pressure created under fast-moving vehicles sucks dust from the road surface, creating dust clouds and a gradual unraveling of the road material. This problem was approached by spraying tar on the surface to create tar bound macadam".
The stretch of roadway before you contains several examples of cinder use in the road building process. Though only visible in a few locations the base material contains cinders, along this stretch you can observe patch materials of asphaltic concrete as well as paved shoulders, note the red hue from the cinders in the shoulder. EDIT! In 2017 the road was overlayed and the old red patches were covered with modern day Asphaltic Concrete.
So as Route 66 moved across the country and folks enjoyed their trips (and Burma Shave signs) they may or may not have known they were driving on pieces of an old volcano.
He played the Sax,
Had no B.O.
but his whiskers scratched
so she let him go.
Try Burma Shave.