The fold and thrust belt of northwestern Montana has been the subject of many structural and paleomagnetic studies, which indicate both counterclockwise and clockwise rotations of thrust sheets. To further delineate the boundaries of diverging rotations and the nature of emplacement of thrust sheets, structural and paleomagnetic data were collected from the footwall of the Lewis-Eldorado-Hoadley (LEH) thrust plate. This thesis is a test for rotation about a nearby vertical axis accompanying thrust emplacement in the northern Rocky Mountains based on structural and paleomagnetic analysis. The area of interest was mapped for geologic relations; bedding attitudes, simple folds thought to be associated with thrusting, and fold plunges. Research shows a trachyandesite sill and adjacent sedimentary units in the area are folded with a slight vergence to the east and plunge ~17^o NNW. The sill yields a 40Ar/39Ar biotite cooling age of 58.8 +/- 1.5 Ma, but is interpreted to have a crystallization age of 75.9 +/- 1.2 Ma (prethrusting) from stratigraphic, petrographic, and structural constraints. Fifty-six oriented hand samples were collected in the field and at least one core drilled from each sample. A paleomagnetic investigation yielded a stratigraphic mean direction of Dec = 200.0° and Inc = -62.3° , with a virtual geomagnetic pole located at 75.5° N, 166.2° E. A fold test applied to complete unfolding failed at the 95% confidence level. However, progressive tilt correction of the sill suggests a positive fold test at the 95% confidence level for 80% unfolding. Yet statistical analysis yielded the highest precision and smallest a₉₅ at 70% of unfolding of the sill. This may indicate the Late Cretaceous sill intruded into partially folded strata in the footwall of the LEH plate as thrusting continued to the east, and that deformation of the Montana disturbed belt began before 76 Ma. Reference directions from the Late Cretaceous to Eocene are undistinguishable at the 95% confidence level. A comparison of the observed direction to the expected direction for the Late Cretaceous yields a 38° +/- 13.2° clockwise rotation about a nearby vertical axis.