Introduction to Geology

The following chapters are an introduction to the geology that is relevant to the geologic maps of the Middlesex Fells Reservation. It is intended to give non-geologists an introduction to the subject in hopes of making the maps more understandable.
Links are the title captions of each chapter.
Written by: Jack Ridge, Dept. of Earth and Ocean Sciences at Tufts University
First online: August 30, 2021
Usage should be credited: Jack Ridge, 2023, Geology of the Middlesex Fells, Middlesex Fells (
Featured image above: Eastern side of the Sierra Nevada Mountains viewed from the Alabama Hills near Lone Pine, California

Introduction, Preface and Acknowledgements
This chapter introduces you to the project at Tufts University to make geologic maps of the Middlesex Fells Reservation. It gives the motivations for mapping the Fells and the importance of understanding the geology of this area. It explains what starting knowledge you must have to understand the fundamental geological science that is discussed in the following chapters. At the end of this section are acknowledgments to people who helped along the way.

Chapter 1: Geologic Maps
As a starting point, this chapter introduces you to geologic maps, the information they portray, and how they are made.

Chapter 2: An Introduction to Earth Materials
Before you can understand the geologic history of an area you must have a fundamental understanding of what Earth is made of: rocks. The purpose of this chapter is to provide you with introductory knowledge of the materials that make up Earth’s crust. It does not provide a detailed discussion of rocks specific to the Middlesex Fells, which will come later, but focuses on an introduction to the building blocks of rocks, what we call minerals, and the fundamental rock types.

Chapter 3: Minerals – The Building Blocks of Rocks
This chapter introduces: 1) mineral groups and the properties that allow us to distinguish different minerals; 2) some common rock-forming minerals in the Fells that are also common elsewhere; and 3) some not so conspicuous minerals in the Fells that are important to understanding its geologic history.

Chapter 4: Igneous Rocks
Most rocks in the Fells are igneous rocks. This chapter will introduce you to specific types of igneous rocks and how they form.

Chapter 5: Sedimentary Rocks
This chapter is a summary of sedimentary rocks and their terminology. Most of the sedimentary rock units in the Fells are secondary to the igneous rocks in the previous chapter and have mostly been lightly metamorphosed. As a result, this will not be a comprehensive treatment of the subject.

Chapter 6: Metamorphic Rocks
      This chapter describes processes involved in metamorphism and the characteristics of metamorphic rocks.

Chapter 7: Deformation of Rocks – Structural Geology
In this chapter you will learn about some of the structures that are produced when rocks are deformed by stresses in the crust.

Chapter 8: Geologic Events and Time
This chapter focuses on determining the relative ages of rock units from field relationships and how we put together a geologic history or sequence of events. You will also learn how we determine the numerical ages of rock units.

Chapter 9: Earth’s Interior and Plate Tectonics
In this chapter you will learn about what drives mountain building, causes regional metamorphism, generates magmas, and forms ocean basins, a system known as plate tectonics.

Chapter 10: Surface Processes
In this chapter you will learn about some of the processes that recently shaped Earth’s surface through weathering and erosion. A big part of this in New England is glaciation and past climate change.

Glossary of Terms
This document is a glossary of terms from the introductory chapters above.