Continuing down the Marathon Basin on Hwy 385, the southern margin on the left of the highway is seen in the above photograph, taken 24 miles south of Hwy 90. On this margin, the Cretaceous strata dip only very slightly to the south, perhaps because they are a distance from the center of the uplift. The knob in center is a small intrusion. Several similar intrusions occur along the margin.
A close-up view of Santiago Peak from the south, 50 miles down Hwy 385 from Hwy 90. The northeast wing of the intrusion can be seen from this angle, about 6,250 feet at the peak, 270 feet lower than the main body. The shapes of the two summits suggest that they are part of a plug, not an eroded sill. The flat top seen in the previous photo would therefore be the result of the uprising intrusion coming in contact with a resistant rock layer, perhaps a thick lava bed.
It is hard to believe that there were lava beds at this altitude when there are no traces of volcanic rocks of the main volcanic phase east of this point, but Elephant Mountain, 12 miles to the north is definitely a sill, the top of which is at 6,230 feet, so there has been a great amount of volcanic material removed in the last 30 million years.
Continuing down Hwy 385 in the Marathon Basin, one of the most spectacular sights are the flatirons on East Bourland Mountain, photographed 8 miles south of Hwy 90. A flatiron is a short, triangular hogback forming a ridge or spur on the flank of a hill that looks like a flatiron. A flatiron is usually a plate of steeply inclined resistant rock, in this case Caballos Novaculite, called a flatiron from its shape. A flatiron is a short, triangular hogback forming a ridge or spur on the flank of a hill that looks like a flatiron. Here, the light-colored flatirons of novaculite stand out against dark Maravillas Chert on the higher parts of the mountain. The Maravillas Chert Formation is of bedded black chert and dark gray to black limestone, 100 to 400 feet thick, the next youngest to the novaculite in the Marathon Basin succession.