The periodization used to distinguish sites and artifacts as "prehistoric" or "historic" translates to the selection of field methods and analytical techniques. This comes at the expense of developing new approaches to track continuities and adjustments in Native American site use, technologies, and other cultural traditions, such as mobility across an artificial divide between prehistory and history. To evaluate the mobility of Coast Miwok people in colonial San Francisco Bay, California, this article presents an experimental technique that compares radiocarbon and geochemical data from a Late period Phase 2(A.D. 1500-1800) shellmound (CA-MRN-114) to baptismal records from Spanish missions (A.D.l 776-1830s). Supported by eyewitness accounts of native fugitivism, furlough, and foraging at the missions, Coast Miwok baptisms before 1817 are at their lowest during traditional times of mussel harvests. After 1817, a different pattern is examined vis-d-vis the colonial landscapes taking shape in the region. Radiocarbon, geochemical, and documentary evidence supports the conclusion that seasonally oriented Coast Miwok mobility involving the collection of shellfish continued even during missionization. With further refinement, the proposed methodological framework holds promise for documenting patterns that often go unseen in the historical record and enhancing the archaeology of colonialism in North America.