Sur Lies is a French term meaning, literally, 'on the sediment'. In winemaking, this generally refers to the sediment of the grape that ‘falls out’ of the juice after pressing. There is a significant difference between the grape lies and the so-called yeast lies, a point which will be discussed later. Aging sur lies is a time-honored practice in the history of winemaking (and brewing - the Germans call the process laagering).
The easiest way to explain the lies is by using the example of a jug of apple cider that you might buy at the grocery store. The apple cider, over time, has a sediment layer that settles at the bottom of the jug. If you were to taste the clear juice from the top of the jug, it would taste a lot like clarified apple juice with nothing much going on. However, if you were to shake up the jug and unsettle the apple lies from the bottom and then taste the juice, you would have a much more complex apple flavor that would explode in your palate. This phenomenon is due to the fact that most of the complex flavor in the apple (and the grape) is contained in this sediment.
Now, since we know the real flavor is in the sediment, how do we go about getting it out? Simple. We add the fourth dimension - Time. The wine is literally aged 'on the sediment'. The length of time the wine is aged sur lies directly affects the flavor of the finished product. This can be equated to cooking a good sauce. Most chefs will tell you that cooking a good sauce takes time (sometimes days) to release the complex flavors of the ingredients into the sauce. In winemaking, this process can take months and sometimes years, depending on the style of the wine. The longer the wine is aged sur lies, the fuller and more complex the wine will be. Additionally, the longer it takes a flavor compound to release from the lies, the longer it will take to release on the palate. As a result, sur lies aging succeeds in making long, complex, layered wines. Historically, a standard Burgundian wine would be aged for up to a year sur lies. Only the top Domaines would take the time to age them two years or more. At Blackwood Canyon there are currently wines still on the lies after fourteen years.
This brings us back to the distinction between grape lies and yeast lies. As the terms would suggest, grape lies are the grape sediment, whereas yeast lies are made up essentially of the yeast sediment left after the fermentation of juice which has been previously 'racked' from it's grape lies. There is a large and growing sector of wineries claiming to be aging wine on the lies, when (unfortunately) what they are really doing is aging the wine only on the yeast sediment for a short period of time (a few months is extreme). Intuitively, one can understand that there is little if anything to be gained, in terms of flavor extraction, by aging a wine longer on the yeast sediment only.
What's worse, the yeast sediment does not provide the protection from oxidation that the grape lies would, making it necessary to protect the wine from oxygen in other ways (sulfiting being the most common). An interesting and important secondary effect of lies aging is the ability of the grape lies to naturally protect the wine from oxidation. The lies form a naturally reductive environment in the wine that renders heavy-handed techniques such as over-sulfiting, which can be harmful to the final product, unnecessary.