During the last two years a series of deep excavations have been made in the floor of the Aire valley near Skipton (on the border of sheets 92 S.E. and 92 S.W. 1 inch to 1 mile, Geological Survey). These have brought to light a hitherto unsuspected deposit of laminated clays of great thickness, which are of considerable interest in interpreting the events of the Glacial retreat in this valley. A. Jowett and H. B. Muff in 1904 published their work on the glaciation of the Aire valley between Keighley and Bradford, in the course of which they described three terminal moraines of the Aire glacier, at Tong Park, near Baildon, at Hirst Wood, about a mile and a half west of Shipley, and at Bingley; in the course of the paper they mention without comment the fact that above the Hirst Wood and Bingley moraines there is in each case an extensive flat of alluvial material, extending far up the valley. Although these have long been tacitly regarded as lake flats, the first confirmation was received in 1920 when the excavation of a deep trench in the flat half a mile west of the Hirst Wood moraine revealed a quantity of laminated clay underlying the undoubted river gravels. The excavation was only temporary, and unfortunately no measurements or deeper trials were obtained. Recently a trench above the Bingley moraine has been cut deep enough to show the laminated clay underlying the river gravels here as at Hirst Wood, and these two exposures have directed attention to the existence during late Glacial and post-Glacial times of a series of lakes held up by the terminal moraines left by the Airedale glacier. That such lakes existed has long been the opinion of most local geologists, but they have always been regarded as of brief duration, drainage being rapidly accomplished by breaching of the moraine barrier, or, in the Hirst Wood case, by super-deepening of the cut round the north end of the moraine, a cut probably initiated in late Glacial times. The material from the Skipton excavations indicates that these lakes were of far longer duration than previously suspected.