Smaller lakes known as tarns occupy glacial cirques at higher elevations. Glacial erosion produced numerous granite outcroppings, tarns, cirques and knife-edged ridges. Numerous smaller lakes, known as tarns, are located in cirques formed by glacial erosion. Hows area originally contained three much smaller tarns, Low Tarn, Middle Tarn and High Tarn. Many glacial cirques contain tarns dammed by either till (debris) or a bedrock threshold. Smooth sweeping slopes predominate with a minimum of tarns or crags. Other routes from Langdale climb via Rossett Gill and Three Tarns. There are also a large number of natural lakes and tarns in the upper watershed. Several of these hold tarns, or small cirque ponds. The underlying bedrock is volcanic tuff of the Lincomb Tarns Tuff Formation, overlain with alluvium. Other former tarns have silted up. The summit of Haystacks has a number of tarns. Tarns are found mostly in the upper reaches of the Himalaya, above . The fell is composed of the dacitic lapilli-tuff of the Lincomb Tarns Formation. This is followed by the equally extensive Lincombe Tarns Tuff Formation, which is an ignimbrite.