The Baltimore Mafic Complex (BMC) is described as a layered intrusion(s) consisting of ultramafic to intermediate igneous bodies. Although the complex has been the subject of investigation for nearly one hundred years, uncertainty remains about its origins and tectonic history. This section of hydrothermally altered rock along the Octoraro Creek supports the hypothesis that the BMC is a mafic intrusion into sedimentary rocks, as described by Hanan and Sinha (1989). The section experienced at least two stages of deformation: the first at high temperatures associated with intrusion creating grain boundary migration and subgrain formation, and the second hydrothermally altering remaining minerals. These features indicate that the origin of the Baltimore Mafic Complex is not affiliated with a subduction or supra-subduction zone ophiolite.