Historical Geology/Lakes

In this article we shall look at the characteristics of lakes and how we can recognize former lakes in the geological record. The reader will find it useful to be familiar with the previous article, on deltas, before reading further.

Lakes
A lake is an inland body of water fed by rivers, streams, or sometimes by seepage of groundwater, as in deflation lakes in deserts. There is no universally agreed distinction between a lake and a pond, so you may as well think of a pond as being a small lake.

A large lake may be influenced by the tide, but even a large lake is small compared to the oceans, and the tides are correspondingly smaller; the highest tides in the Great Lakes of North America, for example, cause variations in water level no greater than five centimeters.

Most lakes are composed of fresh water; a salt-water lake can arise when water doesn't flow out of the lake (in which case it is known as a terminal lake) but is instead removed by the water evaporating, leaving behind the minerals that were dissolved in it: examples include the Great Salt Lake of Utah and the Caspian Sea.

The word lacustrine means "related to lakes", and so the sediments associated with lakes are known as lacustrine sediments.

Lacustrine sedimentary rocks: how do we know?
If enough sedimentary rocks are exposed, then we can recognize a former lake by its shape: we'll find a bullseye of terrestrial sediments and fossils surrounding aquatic sediments and fossils, typically with coarser sediments nearer the shore and finer sediments nearer the center.

If, however, only a part of the former lake and its shore are exposed, the sediments and fossils can still provide us with ample indications. If we can find a fraction of the shoreline showing a transition from a terrestrial to an aquatic environment, then that's a clue. But could we not be looking at a transition from land to sea? Well, since lakes are not tidal, or only barely tidal, the shoreline sediments will not show the same tidal effects that we shall discuss in the article on nearshore sediments.

As discussed in the article on deltas, the foreset beds of a lacustrine delta slope at a different angle to those of a marine delta, because the former are produced by fresh water flowing into fresh water. If we can find a delta with steeply sloping foreset beds, we know that the river was flowing into a freshwater lake.

Usually water will flow through a lake, in by one or more streams or rivers and out through others, but since a lake is so much broader than a river, the rate of flow in the lake itself will be small, and the sedimentary structures we associate with the flow of rivers will be small or absent. Varves, on the other hand (as mentioned in the article on glaciers) will often form in the relatively still waters of a lake.

When we look at the fossils in the sediment, aquatic fossils will usually be present; often so will some terrestrial fossils of organisms which got washed into a lake: leaves, for example can easily find their way into a lake, and are often preserved in the lake-bottom mud.

What's more, if, as usual, the lake is a freshwater lake, the fossils will not just be aquatic fossils, but freshwater aquatic fossils; when we find these, we're either looking at a lake or a river, and if the sedimentary evidence rules out a river, then we're looking at a lake.

Note on identifying freshwater organisms
The reader may wonder how we can identify freshwater organisms in the fossil record.

If they are recent organisms, then we can just recognize them. But what about extinct freshwater organisms? Well, if they are recently extinct, they will overlap with living freshwater organisms. Looking at somewhat older rocks, there will be some organisms that we don't recognize, but they will overlap in time and location with organisms we do recognize; if we can identify those as freshwater organisms, then the unfamiliar organisms that lived in the same place must also have been freshwater organisms. And we can continue this line of reasoning: when we find those organisms in company with still more types of fish or shellfish that we don't know, then they too must be freshwater organisms.

We can also look at the sedimentary environments in which organisms are found; fluvial sediments, for example, are very distinctive, and rivers are freshwater except near their mouths, so organisms found in fluvial sediments are freshwater, and if the same kinds of organisms are found in what on sedimentary grounds we would identify as a freshwater lake, then it's reasonable to conclude that it is a freshwater lake.

Then again, we can also use similar reasoning to identify marine organisms. As aquatic organisms are either marine or freshwater, those that aren't marine are freshwater by a process of elimination.