Varve

A varve is an annual layer of sediment or sedimentary rock.

The word 'varve' derives from the Swedish word varv whose meanings and connotations include 'revolution', 'in layers', and 'circle'.

Of the many rhythmites in the geological record, varves are one of the most important and illuminating in studies of past climate change. Varves are amongst the smallest-scale events recognised in stratigraphy.

An annual layer can be highly visible because the particles washed into the layer in the spring when there is greater flow strength are much coarser than those deposited later in the year. This forms a pair of layers—one coarse and one fine—for each annual cycle. Varves form only in fresh or brackish water, because the high levels of salt in normal sea water coagulate the clay into coarse grains. Since the saline waters leave coarse particles all year, it is nearly impossible to distinguish the individual layers in salt waters. Indeed, clay flocculation occurs at high ionic strength due to the collapse of the clay electrical double layer (EDL), which decreases the electrostatic repulsion between negatively charged clay particles.^{[citation needed]}