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The geosyncline did not extend as far to the northwest or southeast. To the northwest the Paleozoic rocks in the nearest exposures are thinner and more calcareous and were laid down on a sea bottom that did not either rise or subside greatly. To the southeast the sediment: in the geosyncline suggest that there was a land area Llanoria, which at times rose to a considerable height In Mesozoic times the Paleozoic geographic feature: had disappeared. The Cretaceous rocks were laic down on the surface of an extensive peneplain carved from the older rocks, and the greatest area of subsidence was southwest of the Marathon region.

The rocks of the region stand in a variety of structural attitudes, and some of them have been greatly deformed. The earlier Paleozoic rocks of the Llanoria geosyncline were folded and faulted before Permian time, and the Permian rocks lie unconformably upon them along the south side of the Glass Mountains. The Cretaceous rocks in turn truncate both the folds of the Marathon Basin and the tilted rocks of the Glass Mountains. The Cretaceous rocks have themselves been deformed, both before and after the early Tertiary volcanic eruptions.

The total thickness of the Paleozoic rocks is about 21,000 feet, of which the pre-Carboniferous strata comprise 2,500 feet; the Pennsylvanian, 12,000 feet; and the Permian, 6,500 feet. The Cretaceous rocks in the area have a maximum thickness of about 1,200 feet. The following table summarizes the formations exposed in the Marathon region: