The Moreno Valley, located along the complex eastern boundary between the Sangre de Cristo Mountains and the Great Plains, is structurally a broad, northwardplunging syncline, disrupted by smaller folds and faults. This major synclinal structure is the result of the Laramide disturbance. Deformation, however, has continued possibly into the Quaternary. Intrusions, probably contemporaneous with those of the Spanish Peaks, have deformed the strata in the northern portion of the Moreno Valley. Scully Mountain is a northwestward-plunging anticline produced by a porphyry intrusion. A Mid-Tertiary surface of low relief is now represented by broad mountain flats at elevations near 10,000 feet. The Moreno Valley was initiated as a topographic feature about the close of the Mid-Tertiary. Its subsequent history is in many respects similar to that of the Rio Grande Depression. During the Quaternary a broad valley stage and three lower pediments have been developed. The lowest pediments are cut on deformed valley fill, believed to be Pliocene and here named the Eagle Nest formation. The early drainage of the valley was south and east into the Canadian River. Stream piracy by the Cimarron River diverted drainage to the east through the present Cimarron Canyon into the Canadian River. Drainage south of the point of capture has been reversed for about 14 miles, the present stream draining the southern portion of the Moreno Valley flowing northward to the Cimarron River. © 1941 Geological Society of America.