The Tarn. Malham Tarn at first glance appears 'out-of-place' in this limestone dominated terrain. The secret behind the Malham Tarns existence can be found in much older rocks that belong to the Silurian Period as the floor of the tarn is composed of Horton Formation (Ludlow). Following the last Ice Age, glacial sediments sealed the tarn's floor thus enabling water to accumulate. Streams flowing from the tarn soon disappear underground when they reach the limestone.
By the way, the North Craven Fault has been crossed by the time you reach the tarn. This has an important part to play in bringing the Lower Palaeozoic Silurian basement rocks into juxtaposition with Carboniferous Limestone. Also note the outcropping Lower Hawes Limestone (Brigantian, Wensleydale Group) forming the prominent scars behind the tarn.