One of the clearest pictures of an old earth comes from carefully looking at the bottoms of lakes. The evidence there gives a very convincing argument that the earth is at least tens of thousands of years old, hundreds of thousands of years old, or even millions of years old.
In many lakes, the sediment at the bottom results from small particles that are constantly settling to the bottom. Over time, these particles slowly build up, making the layer thicker and thicker. Often, the nature of the particles change with the season. The winter months might bring finer particles because there are only weak currents bringing dirt into the lake. But the particles in the summer may be larger, because the water coming into the lake flows much faster due to meltwaters from winter snowpacks. Sometimes other materials get included in the layers. For example, leaves may fall from trees and sink to the bottom during autumn. This creates a thin layer of organic matter.
As these events repeat over many years, patterns form in the growing layer of sediment. This goes on, year after year, so that the bottom of the lake becomes a series of layers, one on top of another.
The layers in the lake bottom become a record of the years that the lake has existed, just like the rings in a tree are a record of the years that the tree has lived.
Scientists study these lake records by drilling into the bottom and extracting a cylinder of the deposits. The layers are often clearly visible, and people can easily tell how many years the process has been going on just by counting the layers. When they do this, they often find lakes with tens of thousands of layers, showing that those lakes have existed for at least tens of thousand of years. Some lakes have even been found with enough layers to show that they are over 100,000 years old. (Ancient lake beds have even been found with millions of layers!)
In lakes that have organic layers, carbon-14 dating can also be used to tell how old those layers are. Those measurements gives the same result that people get from counting the number of layers. (Carbon-14 dating is a scientific measurement that can tell the age of buried plant and animal remains, up to about 50,000 years old.) In other words, two completely different techniques agree on the age of the layers, double-checking that the ages go back more than 30,000 years. This also shows that the layers are due to annual changes, and not some other process.
This simple process show us that lakes can be tens of thousands, or even hundreds of thousands of years old. This means that the earth must be at least that old, but it may be much older. Other techniques show that the earth is much, much older than this, but even this simple picture makes it clear that we need to be careful how we interpret the early Genesis accounts. If we’re not careful, then our interpretations will be based on human perspectives, rather than relying on the voice of God.
Most Young Earth Creationist (YEC) responses to the standard analysis try to generate doubt regarding the standard explanations, but without offering relevant details. For example, some YEC scientists have shown that layers can be formed by completely different processes (ie, hurricanes and volcanoes), then conclude that all standard analysis lakes should therefore be doubted.
Other arguments try to show how multiple layers can form in a single year, but even if that always happened, the ages of some lakes are still quite old. (Check out Old Earth Ministry’s discussion below for more details.)
Because many of the challenges are vague and emotional, instead of detailed comparisons, it would take a series of articles to treat each one.
To learn more about this topic, check out these links.
Here is an excellent discussion of lake varves (the technical term for the layers discussed here), on Naturalis Historia:
Check out number 4 and 5 in this post on Age of Rocks for more technical discussion and links:
Old Earth Ministries has an article which describes varves in a little more detail, and also goes through various YEC arguments more thoroughly: