Many recent lake sediment profiles contain atmospherically derived fly ash and various particles from industrial processes. All these include a magnetic fraction that can be studied by subjecting subsamples to controlled magnetic fields in the laboratory and measuring the isothermal remanences acquired. These provide a basis for partially characterizing and roughly quantifying the magnetic minerals preserved in the sediments. The results presented illustrate those obtained from some 70% of the 39 profiles taken from 32 sites mostly in upland Wales and the Scottish Highlands. They show widespread increases in magnetite and haematite deposition beginning from the mid-nineteenth century onwards and steepening in the last three to five decades.