The shear rate dependence of platelet aggregation geometries is investigated using a combination of in vitro and numerical experiments. Changes in upstream shear rate, γ(Pw), are found to cause systematic changes in mature platelet aggregation geometries. However, γ(Pw) is not the only factor determining the shear rate experienced by a platelet moving over, and adhering to, a platelet aggregation: flow simulations demonstrate that naturally occurring variations in platelet aggregation geometry cause the local shear rate on the surface of a mature platelet aggregation to vary between zero and up to eight times γ(Pw). Additionally, as a platelet aggregation grows, systematic changes in geometry are found, indicating that the local shear field over a growing platelet aggregation will differ from that over mature platelet aggregations.