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... I came across a limestone boulder that appeared to have this same reddish/black material protruding out of the boulder in what appear to be knobs and other shapes. Looking closely, you can see the knobs have in some areas cracked the limestone as they appeared to push out.
GEOLOGY About 40-50 percent of the rocks exposed in the Sandia Mountains are of Precambrian age and include the Sandia Granite (1,445±40 m.y.), which intrudes older biotite schist and gneiss of Kelley and Northrop (1975) and the Cibola Gneiss, and a still older northeast-striking greenstone belt along Tijeras Canyon. The Tijeras Greenstone of Kelley and Northrop (1975), of probable Proterozoic X (Early Proterozoic) age, consists of predominantly dark-greenish-black, blastoporphyritic metabasalt flows, and minor thin lenses of metarhyolite. The absence of pillow structures in the greenstone suggests predominantly subaerial extrusion. The slightly younger Cibola Gneiss is a granitic paragneiss containing intercalated and isoclinally folded quartzite beds. The gneiss and schist of Rincon are highly foliated, biotitic-sillimanitic, metapelitic rocks that have been intricately intruded by the post-orogenic Sandia Granite. Numerous pegmatite and aplite dike swarms that are largely fracture controlled intrude the metapelitic rocks.A relatively thick (2,500 ft) section of Mississippian, Pennsylvanian, and Permian strata rests unconformably on the Precambrian rocks and forms an extensive east-dipping dip slope on the tilted fault block. The Mississippian strata are thin (50 ft), discontinuous, erosional remnants of cherty limestone. The Pennsylvanian section is about 800 ft thick and is represented by carbonaceous shale and sandstone of the Sandia Formation and an overlying thick (650 ft) sequence of shallow marine limestone beds of the Madera Group (Myers, 1973). No attempt was made in this study to differentiate the various units in this group. A complex Permian and Triassic red-bed section of sandstone, siltstone, mudstone, and thin limestone beds crops out farther down the dip slope. From oldest to youngest these are the Permian Abo, Yeso, and San Andres Formations, and the Triassic Santa Rosa Sandstone and Chinle Formation.The remaining Mesozoic strata are folded into synclinal basins, such as the Tijeras and Hagan synclines and also fill ramplike depressions near Placitas (Kelley and Northrop, 1975, p. 83). The Jurassic and Cretaceous sedimentary rocks include the Jurassic Todilto, Entrada, and Morrison Formations and the Upper Cretaceous Mancos and Mesaverde Formations. Black carbonaceous shale and thin coal beds are commonly interbedded with thick sandstone beds in the Mesaverde Formation. Middle Tertiary(?) lamprophyric dikes cut the Sandia Granite along the steep west-facing slope of the Sandia Mountains, and a basaltic dike cuts the Mesaverde Formation about 1 mi northwest of Placitas. The Galisteo Formation of early Tertiary age crops out as an incomplete section north of the wilderness boundary west of Placitas.The dominant structural element is the east- tilted Sandia fault block, which is controlled on the west by the Sandia and Rincon-Ranchos range-front faults. These faults have as much as 20,000-28,000 ft of throw. The eastward tilt of 15°-20°, formed in conjunction with displacement along these faults, steepens where the Paleozoic section underlies the folded Mesozoic sedimentary rocks of Tijeras and Hagan basin synclines.The plunging northern terminus of the fault block is near Placitas, but, to the south, the Sandia fault block is contiguous with the Manzanita and Manzano fault blocks. Numerous north-trending faults along the dip slope are considered coeval with the Pliocene and Miocene Sandia-Rincon- Ranchos faults and commonly displace older faults of post-Permian (principally Laramide) age. Most of the north-trending dip-slope faults are downthrown to the east, but the large Ellis fault is an exception and locally reverses the regional east dip so that near Capulin Peak the strata dip gently to the west. Probably the most important effect of these faults has been to bring Precambrian rocks to the surface along the back-slope between Tejano and Tecolote Canyons.The Placitas-San Francisco, Tijeras and Gutierrez faults strike northeast to east-northeast along the north and south boundaries of the Sandia Mountains. These faults, principally of Laramide age, locally displace the Upper Cretaceous Mesaverde Formation and are commonly branching or splayed fault systems having combined throws as much as 1,000-3,000 ft. The Tijeras and Gutierrez faults bound the Monte Largo horst and the Tijeras graben and syncline; a scissorslike displacement with some strike-slip displacement is characteristic of these faults. Most of the northeast and east-northeast faults have had numerous periods of movement. For example, the Tijeras fault was probably active in the Precambrian with some left-slip displacement (Connolly, 1982), and again during late Paleozoic, Laramide, middle Tertiary,and Holocene times. The Placitas-San Francisco fault system probably had a similar history with the principal displacement taking place during Laramide time.
Firstly, i have to agree with RD- the black "knobs" are more resistant to erosion, so they stick out. The "knobs" are probably very large pyrite nodules. The dark red-brown to black color means they are made of iron and manganese oxide, which I would expect in weathered specimens.