McGookey reported in 2009 that just 17 miles south of Fort Stockton, US Highway 385 crosses this astrobleme. One of Texas' best secrets is that Sierra Madera, immediately to the northeast of the highway, is the central peak of an eight-mile diameter astrobleme. An astrobleme is the scar left on the earth's surface by the impact of a very large meteor.
This astrobleme was thoroughly studied by the United States Geological Survey (Professional Paper 599H, 1972), because it is like astroblemes on the moon and Mars. They are characterized by:
a) an outer rim,
b) a large circular depression, and
c) a sharp peak in the middle.
The USGS found that rocks in this peak originally were more than 4,000 feet below the surface and were rebound from the impact pressure of the large
meteorite. The information gained here, that collecting rocks from central peaks provided samples of rocks from far below the surface, led NASA to program some landings on the moon and the robot Curiosity on Mars to land in the depression near central peaks. The rocks that make up this peak are highly folded and faulted, and have high pressure features, crystals and minerals found nowhere else in the southwest. Astroblems have been found in many parts of theSudbury Complex of Ontaiio (Naldrett, 2003) and the Vredefort ring of South Africa, to the Miocene
age Steinheim Basin of Germany.
Present exposures show a wide range of erosional level. Astroblems have common characteristics that tie them together and allow the more deeply eroded structures, such as Sierra Madera, to be reconstructed by comparison with the astroblems that are less eroded. The structures at Flynn Creek, Tennessee and the Steinheim Basin represent the highest levels of exposure in well-studied astroblems. Both structures have a small central uplift surrounded and partly buried by a lens of breccia that occupies a shallow crater. A somewhat lower erosional level is preserved at Gosses Bluff, Australia, where some highly shocked and melted fallback breccia is preserved, but the original crater walls have been destroyed by erosion.
Sierra Madera represents a still lower erosional level from which most vestiges of a crater-form depression, fallback, and ejecta have been stripped (Wilshire, 1972, p.37). Scaling to the dimensions of Flynn Creek crater and various experimental craters suggests that about 2000 feet of cover may have
been originally present over the Lower Cretaceous beds that has since been removed by erosion (Sierra Madera Figure 4).
Siena Madera was extensively studied in preparation for lunar exploration. These results indicated that one could sample the thickness of lunar crust in the
walls of craters and that rocks from the central uplifts would provide data on subcrater material from depths approximately one-tenth of the crater diameter. The typical lunar crater considered was over 15 miles in diameter. Thus, sampling rocks from the central uplift would allow study of rocks from over 1.5 miles below the surface.
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