What are Glacial Varves?
A varve is simply defined as: an annual sediment layer. Where we see varves today, mostly in lake (lacustrine) deposits, but also in some marine environments, there are seasonal or annual variations in deposition responsible for contrasting layers within one year. Unlike many other environments, preservation and recognition of annual structures in glacial lakes is nearly guaranteed because the activity of organisms (burrowing) is generally very low and does not significantly disturb layers after they form. Thus, varves formed in glacial lakes, or glacial varves, are distinctive features of glacial lacustrine environments. It should be noted that in many places, especially on the internet, varves are frequently defined as a type of glacial lake sediment. This definition stems from the fact that varves are common in glacial lake environments, but this definition overlooks the most important aspect of all varves. Varves are defined as annual sediment layers and they can occur in many different environments.
According to De Geer (1940) the term “varve” in English is derived from the Swedish term “varv”, (“hvarf” in its older spelling), for the cyclic clay deposits underlying the land surface of eastern Sweden.
Varves in Glacial Lakes
In lakes with receding glacial ice margins or that receive meltwater from glaciers, i.e. glacial lakes, seasonal changes in deposition are a function of changes in meltwater and clastic sediment input to the lake between the melting and non-melting seasons. Although it is not strictly true, it has become customary to call the melting season layer of a varve the “summer” layer and the non-melting season layer the “winter” layer. In most, if not all the environments where we find glacial varves, the summer season is shorter than the winter season. In central Alaska, for example, the melting season is typically 3-5 months long while the non-melt season makes up the remainder of the year.
Moreno Glacier, Argentina. Photo courtesy Annie Cardineaux
The melting season in a glacial lake is a time of heavy melting of snow on the glacier and the land surface surrounding the lake. In addition, the melting of glacial ice in the glacier’s ablation zone also contributes large volumes of water. Input of dense meltwater with high sediment concentration triggers circulation and bottom current activity in the lake and results in the deposition of silty to sandy layers. During the summer, ice at the surfaces of most glacial lakes thaws and the lake will have open water for much of the summer when the wind can also cause water to circulate. The thickness of the summer layer will vary from year to year as meltwater discharge and sediment input to the lake vary with weather-controlled meltwater formation.
In glacial lakes, the non-melt season is when surface melting on the glacier and melting of snow on the surrounding land surface come to a halt. As a result, meltwater from these sources also comes to a halt. The only source of water to the glacial lake may be from small amounts of melting of ice at the base of the glacier, a lag in the drainage of glacial meltwater formed over the summer, or a contribution from streams on the land surface where small amounts of groundwater discharge (baseflow) continues to contribute to stream flow beneath thick stream/river ice. A few weeks into the winter season these sources of water represent a very small discharge to the lake as compared to discharge during the summer.
Sediment introduced to a glacial lake in the non-melting (winter) season is also greatly diminished and will be dominated by subglacial sources. As a result of diminished water supply to the lake and the freezing of the lake surface, bottom current movement and wind-driven circulation in the lake are greatly reduced. This allows fine sediment particles (clay-size) to be deposited in the lake for an extended period of time without silt and sand deposition. Thus winter deposition is marked by clay beds. Winter beds tend to not vary much in thickness from year to year, and not nearly as much as summer layers.
Anatomy of a Glacial Varve: What Does a Typical Glacial Varve Look Like?
Passumpsic Valley, East Barnet, VT
Glacial varves have a variety of features in them and vary tremendously in terms of internal grain size and bedding. This is to be expected given the variety of bedrock, physiographic, and climatic regimes where glacial lakes occur. Below is a detailed discussion of what are very typical varves in the upper Connecticut Valley of Vermont and New Hampshire, USA. Like all other glacial varves they have only a small portion of the features that can be found in glacial varves in general. Nonetheless, this is an excellent starting point for understanding the complex relationship between the seasonal changes that occur in glacial environments and the influence seasons have on sedimentation in a glacial lake.
Shown to the right is the typical anatomy of glacial varves in a core from the Connecticut Valley region of northern Vermont. The core has been split, scraped, and partially dried to bring out the contrast between clay-rich beds (dark) that retain moisture and silty to sandy beds (light) that dry much faster. A varve or annual couplet is composed of a light-colored melting season or "summer" layer and an overlying dark-colored non-melt season or "winter" layer. The core section in this image contains 10 winter layers and the varves were deposited about 300 years after the glacier receded. These varves would be considered relatively "distal" to the glacier as compared to varves deposited in the first century of ice recession when the glacier is still close by and the varves are proximal. Most of the annual variation in thickness is due to summer layer thickness variation. The “summer” or melting season layers are composed of multiple micro-graded beds or laminations that often show a general fining upward and may grade into the “winter” layer above.
A common glacial varve phenomenon is a distinct graded bed of sand and silt that can mark the beginning of the melting season layer (arrows) and is sometimes capped by a thin silty clay bed. It is frequently the coarsest sediment deposited as part of the summer layer. This unit appears to represent the initiation of an energetic current across the floor of the lake that may be triggered by a prominent nival (snowmelt) flood, overturning event, and/or sudden release of glacial meltwater as tunnels and drainage systems are opened in the glacier at the beginning of the summer season. This layer is in sharp contrast to the underlying clay of the previous winter. It is also common in some varve sequences, but not on this image, to see a layer coarser than most other units of the summer marking the top of the summer layer and grading into winter clay. This coarse unit represents a summer-ending event such as a late season precipitation or overturning event.
The non-melting season or “winter” layers (dark) are composed of very fine sediment, usually >90% clay. The winter layers are often gradational below with the top of the preceding summer layer and are sharply truncated above at the base of the following summer layer. Winter beds sometimes contain a fine sand or silt parting that may represent overturning or melting events that occur after the initiation of clay deposition. The recognition of clayey winter layers is the most important part of recognizing annual layering in glacial varves. Winter layers are relatively uniform in thickness as compared to summer layers.