SUMMARY.Diatoms from surface sediment samples from lakes on both sides of a glacial drift border in eastern Minnesota show that Araphidinate diatom genera likeFragilaria, AsterionellaandSynedraare more common in lakes in drift with lower CaCO3whereasStephanodiscusandMelosiraspecies are more abundant in those with high CaCo3. In a lake sediment core from low carbonate driftFragilaria crotonensisandAsterionella formosaincreased when pioneer farmers arrived. These results confirm speculation that high A/C ratios are associated with low‐alkalinity eutrophic lake