Shore bluffs at Mohawk Bay, southern Ontario, reveal, within a complex mélange of glacial sediments, brecciated diamicton surrounding sand-block intraclasts. Typically, each intraclast has an aureole (50–65 cm) of brecciated diamicton. It is thought that a causal link exists between the origin of the intraclasts and the deposition of the diamicton. A hypothesis is developed in which sand deposits are thought to have been frozen, eroded, entrained, and eventually deposited within a deforming subglacial or glacio-aquatic debris layer. These sand blocks remained frozen after being deposited, forming intraclasts encased within the diamicton. It is proposed that differential cryostatic stresses developed at the interface between the frozen blocks and the diamicton, causing tensile stresses to develop over short distances and fracturing (brecciation) of the rapidly cooling diamicton. To test this hypothesis, a simple heat-flow-simulation program (COLDSPOT) was run using parameters akin to those found at Mohawk Bay. Results from the simulation are compatible with field observations. The findings from Mohawk Bay have significant implications for glacial sedimentology and glacier bed conditions.