Digital Varves

Varve Coding System

The numerical coding of varves is the description or classification of varves using two-digit integers, which lends itself to non-parametric statistical and graphical analysis of varve sequences. The coding system is a way of evaluating the frequency of varve features and how they might change both temporally in a single varve section and spatially when comparing different correlative varve sections. Changes observed in varve sequences allow an analysis of processes related to ice recession or readvance, climate change events, and the transition from glacial to non-glacial environments. If nothing else, the numerical coding system forces the user to make observations on the internal structure of varves and to look for features that might otherwise get overlooked.

The coding system covers summer and winter layer characteristics as well as special features. Here, as in most glacial lacustrine varve studies, the “summer” layer refers to the silty clay to sandy melt season layer and is the bottom unit of a varve’s annual couplet. The “winter” layer refers to the upper, usually very clayey, non-melt season layer of the varve. The most important aspect of glacial varve analysis or counting is the recognition of distinct nearly pure clay beds as winter layers along with the overall pattern of bedding and grain size in the varve that help define annual layering.

The coding system was developed based on observations of varve features mostly in the Connecticut and Merrimack Valleys of New England and the Champlain and Mohawk Valleys of New York as well as minor observations in a few other areas.

Identifying features with the coding system relies heavily on visually recognizing the contrast between light and dark units in partially dried cores and using darkness contrast as a relative indicator of clay content with lighter tones representing coarser sediment and darker tones representing finer sediment. This works well on gray varves but may not always work with other colors or in varves with mixed colors, especially where red sediments are involved. Cores must be partially dried and scraped carefully for a clear view or thin sections must be prepared (not logistically reasonable for thick varves) to facilitate accurate identification of intra-annual features!! The system will not work on wet or very moist cores because moister tends to obscure contrasts in grain size that allow the detailed identification of subtle varve features in the summer layer according to grain size.

Coding System Downloads

Below are the components necessary to code varves. All documents below were last updated August 1, 2008.

  1. Coding Instructions and Key to Numerical Code Values – PDF document including definitions of numerical code values and instructions with a flow chart on how to code.
  2. Icon Key to Numerical Code – PDF document including icons for code values in a menu format.
  3. Measurement Code Card – PDF document including icons and definitions for code numbers used to indicate measurement uncertainties (numbers 90-99 of the code).
  4. Pictorial Guides to Numerical Code (~9 MB)– PDF document with example images of varves and their code values.
  5. Coding Spreadsheet – EXCEL spreadsheet for coding varves.