The Ancestral Rocky Mountains were intracratonic block uplifts that formed in Colorado and the surrounding region during Pennsylvanian time. Their development was related to the collision suturing of North America with South America-Africa, which also resulted in the Ouachita-Marathon orogeny. In Early Pennsylvanian time, suturing was taking place only in the Ouachita region, and foreland deformation took place largely in the midcontinent. By Middle Pennsylvanian time, the length of the active suture zone had increased, extending from the Ouachita region to the Marathon region. At this same time, deformation of the craton also increased in intensity and in areal extent, culminating in the Ancestral Rocky Mountains. In Late Pennsylvanian time, suturing was taking place onl in the Marathon region, and cratonic deformation decreased areally and spread southward into New Mexico and western Texas and westward into the Cordillera miogeocline. The Ancestral Rocky Mountains and related features in a broad area of the western United States were formed while an irregularly bounded peninsula of the craton (including the transcontinental arch) was pushed northward and northwestward by the progressive collision suturing of North America and South America-Africa. This intraplate deformation was, in some respects, like the Cenozoic deformation of Asia in response to the collision with India.