Geology Rocks!
The Harrison Yocum Collection
Be Sure to Look Down!
Most garden beds are lined with rocks from the vast Yocum Collection. Below is a list of specimens you will see as you walk through the Cactus & Succulent Garden.
Thank you to the University of Arizona Alfie Norville Gem & Mineral Museum for their assistance in interpreting the Yocum Collection. For more information click here.
Plants, Rocks, and Harrison Yocum
Some plants, known as indicators, can reveal if the soil in which they grow is rich (or poor) in certain minerals or nutrients, and/or specific soil conditions such as acidity. The plant may accumulate a mineral or nutrient in its tissue, or its color may change in the presence of acids or bases, like a hydrangea.
Silver
- Lonicera confusa (soft-leaved honeysuckle)
- Eriogonum ovalifolium (cushion buckwheat)
Copper
- Eriogonum ovalifolium
- Eschscholzia californica (California Poppy)
- Penstemon parryi (Parry’s penstemon)
Zinc
- Viola calaminaria (zinc violet)
Limestone
- Fouquieria splendens (ocotillo)
Who was Harrison Yocum?
Harrison Yocum (1923-2010) was a respected horticulturist and one of the founders of Tucson Botanical Gardens. He was a mentor to geology students, fondly referred to as ‘Rock Hounds‘, leading them on geology field trips throughout Arizona and New Mexico.
Obsidian & Basalt Clinker
While they look similar, the Clinker was likely collected from mine tailings. They are denser than volcanic basalt and show a spectrum of color, not just dark grey and black. Obsidian is a natural silica-glass made by rapidly cooling magma. You can see conchoidal fractures, the circular features you can see on the surface.
Pyrite & Sulfides
Pyrite is a common sulfide mineral (FeS2) commonly known as Fools Gold. It is formed hydrothermally, through the movement of hot water and gases within or on the Earth’s surface, and is found in many different types of rocks.
Limestone
Limestone is a very common gray sedimentary rock formed in shallow sea environments. Tucson was once the bottom of an ancient ocean and the limestone often contains fossils and impressions of ancient sea life.
Calcite & White Rocks
The white rocks contain calcite, agate (quartz), and speleothems, unique cave formations made from mineral deposits in groundwater. Some of the specimens may have formed in a local Southern Arizona cavern.
Sandstone
Sandstone is a sedimentary rock composed of solidified grains of sand. You can see layers of sand grains, likely composed of quartz, feldspar, and some iron, giving it some gold and orange color.
Pink Volcanic Rhyolite
Rhyolite is a generally pink volcanic rock that contains quartz, potassium feldspar, and mica. Looking closely at the rhyolite boulders, you can see many angular pieces of other types of rock.
3 Types of Rock
Igneous: Formed when magma or lava cools and solidifies.
Sedimentary: Formed when particles settle out of water or air, or when minerals precipitate out of water.
Metamorphic: Formed when existing rocks are changed by heat, pressure, or reactive fluids.
Calcite & Agate
These rocks contain calcite, lace agate, and quartz. A few specimens show lime boxwork, which is a cave formation made of calcite that forms a unique lattice/box pattern.
Quartz
Quartz is a very common mineral made of silica and oxygen (SiO2), that is found in igneous, metamorphic, and sedimentary rocks. It grows in hexagonal crystals, often in groups or clusters on rocks, sometimes with a pointy termination at the top.
Small to medium-sized cavities inside, coated in small quartz crystals are known as vugs.
Basalt
Basalt is a volcanic rock that forms when lava cools very quickly. This basalt is vesicular, meaning it has holes from escaping gas as it cooled.
Rhyolite
Rhyolite is an extrusive igneous rock, formed as a result of a volcanic eruption. It is composed of quartz, potassium feldspar, and biotite.
Greenschist with Quartz Boudins
A very unique metamorphic rock that formed under regional metamorphic processes, such as lower temperatures and pressures. It gets its green color from chlorite, actinolite, and epidote. Prior to metamorphism, this rock had round quartz fragments, or clasts, which have been squeezed and stretched through the rock.
Conglomerates
These rocks may look manmade but are actually sedimentary rocks composed of weathered, rounded clasts naturally cemented together. The round parts are rocks that were smoothed and tumbled in rivers, and over time stuck together to form these unique specimens.
Copper-Bearing Minerals
Rocks that contain very bright green, blue, and teal-colored minerals, indicate that copper (Cu) is present. Azurite (blue), malachite (green), and chrysocolla (teal) are all local southern Arizona minerals that contain copper.
Agate
Agate is a type of chalcedony or a very fine-grained variety of quartz. It forms in bands that can vary in color, shape, and transparency.
Dendrites
Dendrites are branching structures of Manganese (Mn) that grow along fracture planes in rocks. They may look like fossilized plants, but they are formed from manganese-rich water flowing through the rock.
