The Wonderful World Of... Blog

The Wonderful World of Geology

The Wonderful World of… is a monthly blog focusing on some of the many themes, concepts, and sub-disciplines related to the fascinating world of Southern Californian archaeology.

What is Geology?

While geology is a scientific field of its own, archaeology and geology are very much intertwined. An incredibly important part of geology is the study of rocks and minerals! Rocks and minerals have been useful to humans and animals since the very beginning. Utilized as tools, building materials, shelter, and even consumed daily, these natural resources have been and still are important and beneficial pieces to life as we know it.

Learn about the differences between rocks and minerals in this video.

Depending on their formation, rocks are categorized into three basic types: Sedimentary, Igneous, and Metamorphic.

Sedimentary rock
Photo: Rhododendrites

Sedimentary Rock

Sedimentary rocks are made of minerals that were deposited into the Earth by wind, water, ice, plants, or animals. As sedimentary rocks are buried, their layers are compacted together. A good indicator is of this type of rock is the visible layers of different materials that have been compressed together over time by nature’s elements. There are three types of sedimentary rock: Chemical, Clastic, and Biological.

This is an example of colorful layers of sedimentary rock taken in Makhtesh Ramon, Israel.


Sedimentary Rock in San Diego

Sandstone is a great example of clastic sedimentary rock. This type of rock is commonly found in San Diego County. These rocks are usually very soft, grainy, and easily crumble. Many of the cliffs you see near coastal San Diego are made of sandstone.

Igneous Rock

Igneous rocks, coming from the Latin word ignis meaning “fire”, are formed either beneath the Earth’s surface when magma cools (slowly), forming rocks with large crystals; or on the surface when lava cools (rapidly), and forming rocks with small crystals. Depending on how these rocks cool, they are either considered extrusive or intrusive. Extrusive igneous rocks are formed when lava cools rapidly above ground. Archaeologists often call igneous rocks volcanic rocks.

Photo: James St. John

Igneous Rock in San Diego

Obsidian is an example of igneous rock that is formed when lava cools rapidly. Sources of obsidian in San Diego County likely originate from the Salton Sea, Coso Range, and Baja California. Obsidian is glass-like and popular for projectile points, made by the process of flintknapping.

Photo: James St. John

Intrusive igneous rocks form when magma cools slowly beneath the surface of the Earth. It can take thousands of years before magma solidifies into igneous rock, and the result is a large, crystallized rock. Think of the large granitic rocks that can be seen in places like Yosemite and Joshua Tree National Park.

Granite is a common igneous rock found in San Diego County. Granite is a medium coarse-grained igneous rock that is composed of the mineral’s quartz and feldspar.


Metamorphic Rock

Metamorphic rocks have been changed by heat, pressure, water, or a combination of these elements. Metamorphism literally means to change form. In these extreme conditions, the minerals in metamorphic rocks are replaced and form new minerals. They may look similar to sedimentary rocks but are actually much harder. Common metamorphic rocks include marble, soapstone, and gneiss.

Metamorphic Rock in San Diego

Quartzite is a common type of metamorphic rock found in San Diego County. It is a great example because it is a metamorphic rock that used to be the sedimentary rock sandstone. The sandstone turns into quartzite when exposed to extreme heat and pressure. It is a perfect demonstration of how the rock cycle is always moving and transforming!

Metavolcanic rock

Metavolcanic Rock

Archaeologists use the term metavolcanic when referring to igneous rocks that were subjected to high pressures and temperatures, causing them to change.

Ground stone tools made from igneous rock

Rock Types Used for Tool Making in San Diego

Of the three rock types, igneous rocks are the most predominantly used rock type for tool making in San Diego County. A study done at the SDAC between the years 2007 and 2012 revealed that 66% of the handstone prehistoric cultural resources found in the area were made from igneous rock.

by Jessica McPheters, Collections Manager

Volunteers work in the Center’s Research Library, cataloguing materials and organizing on a computer-based system.

Marketing and Administrative Volunteers assist the Development Office or Administration Office with data entry, updating marketing materials and clerical tasks. Computer experience is a plus.

Docents welcome visitors and answer general questions regarding the Center and exhibits. Docents staff the gift shop and help out with administrative and curatorial tasks.

Volunteers support event activities at the Center, such as the Annual BBQ, lectures, workshops, and fundraising events. Be a part of the party!

Volunteer provide support for K-12 programs offered at the Center. These are fun, hands-on programs that kids really enjoy. Teaching experience is a plus, but not required. Background checks are required.


With the assistance of Center staff, the intern will use photogrammetry to prepare one or more archaeological collections for digital preservation, as well as create a virtual museum exhibit for our Public Archaeology department. Per approval, special projects of the intern’s choosing are also available. During the research and planning, the intern will receive guidance as appropriate to their selected project. Prospective interns should already be familiar with photogrammetric procedures, and Agisoft Metashape.

Collections Management

With the assistance of Center staff, the intern will prepare one or more archaeological collections for curation. During the course of the internship, the intern will learn to identify artifacts and ecofacts common to the San Diego region, including lithics, ceramics, historical objects, and faunal, botanical, and mineral specimens. Center staff will instruct the intern on archaeological laboratory procedures such as basic artifact analysis, manual and computer cataloguing, storage requirements, and preventative conservation. In addition, the intern will become familiar with historical trends in archaeological practice in the San Diego area and will be introduced to current legal and ethical issues in archaeological curation as well as the concerns and rights of culturally affiliated groups with regard to archaeological materials.

Development and Marketing

Under the direction of Center staff, the intern will support the department in various activities, including, but not limited to, fundraising and grant research; e-newsletter development; social media marketing; and website maintenance. This internship will give the intern valuable, real-world experience in non-profit fundraising and marketing.

Geographical Information Systems in Archaeology

Prospective Interns must have completed three courses: Introduction to GIS, GIS Database Management, and Intermediate/Advanced Methods in GIS. The intern will be assigned a project where they will create shapefiles and maps for curated archaeological collections, museum exhibits, and/or public outreach using ArcGIS 10.6. Center staff will instruct the intern on archaeological GIS laboratory procedures such as computer cataloguing, storage requirements, and database management.

Library Science

With the assistance of Center staff, the intern will arrange and catalog materials in the Center’s library. During the course of the internship, the intern will take a leading role in the cataloging, sorting and storing of research files and creating user guides for these collections. Center staff will instruct the intern on archival procedures, computer cataloging, storage requirements, and preventative conservation.

Public Archaeology

Harness your passion for Public Archaeology and gain hands-on experience with K-12 museum field trips, lectures, and public outreach. Additional projects may include creating virtual museum exhibits and activities, assisting in the development and implementation of K-12 curricula programs, planning and presenting public facing content, or educational field trip content of your own design. Must be able to pass a Live Scan.