Paleomagnetic results have been obtained from the Late Cretaceous‐early Tertiary igneous complexes of the north‐central Montana alkalic province. Data from 94 sites in Eocene volcanic and intrusive rocks give a paleomagnetic pole located at 82.0°N, 170.2°E (A95 = 3.5°; k = 18.6), while 36 sites in Paleocene intrusions yield a paleomagnetic pole at 81.8°N, 181.4°E (A95 = 5.4°; k = 20.2). These poles differ by only 1.6° and are not significantly different statistically. The 130‐site virtual geomagnetic poles show no significant elongation and suggest no significant apparent polar wander (APW) during the period of magnetization of the igneous centers. Postmagnetization structural complications in these rocks are minimal. The presence of a single predominant polarity in these intrusive complexes reinforces the radiometric age data that suggest that igneous activity within individual centers was of short duration. The northcentral Montana data together with other early Tertiary, Cretaceous, and mid‐Tertiary paleomagnetic results require modification of our earlier APW chronology [Diehl et al., 1980]. It now appears that APW relative to North America since the Early Cretaceous consists of a polar still‐stand during much of the Cretaceous (120–75 m.y. B.P.), a period of rapid movement in latest Cretaceous time (75–65 m.y. B.P.), and a period of slow polar movement thereafter. The onset of this period of rapid APW correlates well with a major change in plate motions at ∼80 m.y. and the beginning of the Laramide Orogeny.

A total of 505 paleomagnetic samples were collected from 65 sites (volcanic flows) of the Patagonian plateau basalts at four locations in southern Chile and Argentina. K/Ar analyses indicate that 38 flows from two locations form a Late Cretaceous group (64–79 Ma), while the remaining 27 flows are Eocene in age (42–56 Ma). Progressive demagnetization indicates that a characteristic remanent magnetization (ChRM) has been successfully isolated from 59 flows. Rock‐magnetic properties and analysis of ChRM directions within and between sites allow the secure inference that the ChRM is a thermoremanent magnetization acquired during original cooling. The Eocene flows yield 15 independent samplings of the paleomagnetic field. However, these data fail the reversals test and probably do not adequately average geomagnetic secular variation. The Late Cretaceous flows yield 18 independent samplings of the paleomagnetic field, six of normal polarity and 12 of reversed polarity. These data pass the reversals test and have angular dispersion of virtual geomagnetic poles (VGPs) consistent with adequate sampling of geomagnetic secular variation. The mean of these 18 VGPs yields a Late Cretaceous paleomagnetic pole for South America: latitude = 78.7°S; longitude = 358.4°E; A95 = 6.3° (K = 31.6; S = 14.5°). This pole is consistent with similar age poles from North America and Africa when the Atlantic is reconstructed to 70 Ma. Contrary to previous indications, the South American apparent polar wander path does not reach the present rotation axis until after Late Cretaceous time.

The apparent polar wandering path from Late Cretaceous through early Tertiary time is not well defined. We have collected 33 paleomagnetic sites from the Paleocene Moccasin, Judith, and Little Rocky Mountains and from these a North American Paleocene paleomagnetic pole position has been determined at 174.9°W long., 80.5°N lat., with a circle of 95% confidence of 5.6°. This pole location serves as a Paleocene reference pole and enhances the North American apparent polar wandering path for a critical period of its history.

Paleomagnetic poles have been determined from two Early Eocene igneous centers located in the north‐central Montana alkalic province. The mean paleomagnetic pole for the Highwood Mountains lies at 81.2°N, 167.3°E (N = 29; α95 = 7.1; k = 15.4) while the corresponding pole for the Bearpaw Mountains is located at 80.5°N, 198.4°E (N = 18; α95 = 5.8; k = 37.0). Combining these results with studies on Eocene rocks from Wyoming, gives a reference pole located at 81.7°N, 171.2°E with a circle of 95% confidence of 4.4°. This data, when compared to other early Tertiary results, the Cretaceous stillstand poles and the well grouped mid‐Tertiary poles from the continental interior, suggest that for the period 120 to 15 m.y.B.P. apparent polar‐wandering for North America occurred as a succession of rapid shifts of the geomagnetic pole separated by periods of stillstand.