The rocky, photic benthos of Arctic and Subarctic Biogeographic Regions has a characteristic seaweed flora that includes an extensive high-magnesium calcium carbonate basal layer of crustose coralline red algae. The thickest (10–40 cm) and oldest parts of the crust (previously reported as up to 640–830 years old), primarily at mid-photic depths of 15–25 m, are composed of buildups of the genus Clathromorphum. Due to its annual growth increments and cycling of Mg content with temperature, Clathromorphum has recently been developed as a high-resolution climate archive. The age of the archive is primarily limited by the boring of mollusks that reduce structural integrity, remove the record, and induce local diagenesis. Depressions and gentle slopes in the deeper portions of Subarctic rocky bottoms often collect mixed bioclastic and siliciclastic sediments, including a dense cover of rhodoliths (Lithothamnion glaciale and Lithothamnion tophiforme). In this paper we describe a transition zone of these two environments that forms on cobble/boulder glacial erratic bottoms in northern Labrador. Clathromorphum compactum buildups on the boulders and cobbles projecting through rhodolith beds can be preserved by fine-grained anaerobic sediments that in turn reduce mollusk boring. This significantly enhances preservation and longevity of C. compactum crusts. We describe specimens of ages up to 1200 years BP, and discuss how greater ages can be obtained for archiving high-resolution climate information.