Ostler lenses; possible environmental indicators in fluvial gravels and conglomerates

The Ostler lens is a sedimentary structure observed in recent and ancient coarse gravelly sequences. In recent environments they are small dunes, mounds or small transverse bars made up of rounded, sorted small pebbles. They have been observed in streams carrying predominantly coarse gravel to fine pebble materials, usually in shallow reaches at the downstream end of braid islands, bars and point bars. The Ostler lenses are considered to record essentially unidirectional to slightly convergent flow and a turbulent lower flow regime conditions for the riffle (shallower) parts of alluvial channels.