Some time ago I dreamed about the perfect tool for teaching historical geology. It would be an interactive timeline that students could update with events, and it would even have the ability to allow students to quiz themselves.
While that tool has yet to materialize, I did come up with a low-tech solution that permits the plotting of geological events that occur over widely varying timescales. I created four timeline forms to fit 8.5” x 11” paper. The forms cover different intervals and scales of geological time. They are:
- Timeline 1: The Precambrian (4.5 billion years to 0.5 billion years ago
- Timeline 2: The Paleozoic Era (540 million years to 250 million years ago)
- Timeline 3: The Mesozoic and Cenozoic Eras (250 million years ago to present)
- Timeline 4: The Cenozoic Era (last 10 million years of the Cenozoic Era)
The forms have divisions of the geological timescale along the bottom, and a numerical timescale across the top.
So here you go, but you should probably know…
Plotting geological events on the forms can be tricky. Some events have well-known dates, but some don’t. Events may occur in a geological instant, or take hundreds of millions of years. I provided a sample timeline to give students an idea of what to do in different cases, and how to express uncertainty.
Even with those considerations handled, the students who used this exercise were looking up dates for themselves, and that added another layer of complexity. I prepared a handout to guide them through some additional challenges.
Challenge 1: Different sources or different pages in the textbook give more than one date for an event.
Expect that this will happen. Sometimes a source will be general (akin to saying World War 2 happened in the twentieth century), and sometimes it will have more specific information (it began in 1939). Ideally, the interval on the timeline should reflect the most specific information available (i.e., the most specific date or the narrowest range of dates). This makes it much easier to see how the timing of one event compares to the timing of another. We could plot the appearance of both the Ford Model A and the Ford Mustang as single bars covering the twentieth century. But if someone who had never seen or heard of an automobile before looked at our timeline, they would lack the context to understand that some evolution had taken place.
Keep in mind that geologists themselves may not know the exact dates of a particular event. The boundaries of the timescale can also shift if new information comes to light, so an older source might put the same event at a slightly different time than a new one.
In the end, students shouldn’t get hung up on finding exact dates (because mostly they won’t be able to), but they should be trying to get the dates in the right ballpark, and trying to make that ballpark as small as possible.
Challenge 2: The textbook or other source mentions a geological time interval that isn’t at the bottom of the form.
The forms have increments of the geologic timescale (eras, periods, and epochs in some cases) at the bottom, but these intervals are in fact subdivided into smaller ones. For example, the Devonian period is divided into Early, Middle, and Late epochs. The Late Devonian is divided into the Frasnian and Famennian ages. To figure out where a division belongs, consult the Geological Society of America’s Geologic Time Scale.
Challenge 3: The source says an event happened in the “upper” or “lower” of some interval.
When talking about the timescale, periods are sometimes divided into Early, Middle, and Late. These are official designations for the timescale. In contrast, when talking about the rocks themselves, a layer may be referred to as “Upper Devonian.” “Upper” and “Lower” refer to the position of a bed in a stack of rock layers. Older beds are lower down, and younger beds are higher up. While careful decisions have been made about the ways in which uppers and lowers fit into the timescale, a rough approximation for this exercise would be to treat “Upper” the same as “Late,” and “Lower” the same way as “Early.” A less-rough approximation would be to look up the ages of the units in question.
Ways to use the timelines
My students used the timelines to plot events listed in assignments. Some students printed the sheets and wrote directly on them. Some students added to the timelines using drawing software, or using the mark-up tools within Adobe Acrobat. But if you wanted to go all out, you could print the forms out in mega-huge format, post them on the classroom wall, and stick on images or text to mark events.
Or you could turn them into PowerPoint backgrounds and have students build presentations on top of them.
Or you could make them SUPER-mega-huge in drawing software, decorate them all up with whats and whens, then print a poster.
Whatever you do, send pictures!