This study uses magnetic surveying and modeling to determine the geometry of a mafic sill intruded into Cretaceous limestone at Dagger Mountain in Big Bend National Park, Texas. Magnetic data was collected across the sill and modeled as either flat, folded, or ring dike shapes. The flat sill model provided the best fit to the observed magnetic data, suggesting the sill was emplaced after Laramide folding of the surrounding rock. This supports intrusion around 31 million years ago during Basin and Range extension rather than forceful emplacement during the Laramide orogeny as previously thought. Further geophysical surveys and radiometric dating are needed to fully understand the emplacement of sills