Geologic history is the key to this guide and to understanding the story recorded in the rocks of the region. The subsequent topics revolve around this central chapter. By understanding the historical context of the rocks and geologic processes observed in the Northeast region, we can make sense of geology. The rocks in your backyard fit into a much larger story of shifting plates and colliding continents. By knowing more about the geologic history of our region, you can better understand the type of rocks that are in your backyard and why they are there. Rather than focus on specific periods in geologic time, we will look at the history of our region as it unfolds: as a series of major events over the past one billion years and how those events created and shaped the Northeast. These events will act as the framework for the topics to follow and will shed light on why our region looks the way it does! 

Geologic Time

Figure 1.1: Geologic Time Scale (not to scale).

How did geologists come up with a timeline for the history of the Earth? Over the course of many years and through the combined work of geologists around the world, the Geologic Time Scale was developed. There is not a rock record in any one place that has the complete sequence of rocks from Precambrian to the present. Geology as a science grew as geologists studied individual sections of rock. Gradually evolutionary successions of fossils were discovered that helped distinguish the relative ages of groups of rocks. Rock units were then correlated with similarly-age rock units from around the world. The names you see for the different periods on the Geologic Time Scale have diverse origins. Time periods were named after mountain ranges and even ancient tribes like the Silures of England and Wales, from which the "Silurian" Period was derived.