Our Desert Home: Shadow Mountain Golf Balls
Some of the oldest rocks in California are found in the Shadow Mountains, ranging in age from about 550 million to about 400 million years before present. These rocks have been subjected to numerous episodes of our earth’s dynamics. The latest episode which stretched, compressed, recrystallized, folded, and intruded these rocks happened about 70 million years ago when the hot younger igneous granitic rocks intruded the older rocks. Episodes of deformation complicate the geologist’s ability to decipher what the rocks can tell us by overprinting some chapters, but metamorphic processes can also sometimes form some pretty spectacular mineral species. Extreme metamorphism in the Shadow Mountains has formed large garnets up to golf-ball size and besides being cool to see, they can tell us at least part of a geologic story too!
The Shadow Mountains are located west of Highway 395 about 10 miles north of Adelanto. The mountains form a sort of horseshoe that faces to the east (see the photo). The Shadow Mountains are divided into three distinct areas: the southern, the central and the northern. The “golf ball” garnets are in the northern part.
Garnets are pretty useful minerals. Probably most of us know them as a dark red or reddish purple semiprecious gemstone; the birthstone of January. Garnets are hard (6.5 to 7.5 on the Moh’s scale) and pretty heavy (specific gravity 3.5 to 4.3). They form mostly 10 sided crystals, or even 12 sided crystals, ranging in size from millimeters to, well, golf-ball size.
Garnets are composed of some of the most common elements in the earth’s crust yet are some of the most diverse of the gem minerals with varied chemistry resulting in a variety of colors, form, and physical properties. Garnets are made up primarily of alumina, silica and oxygen, with varying amounts of calcium, chromium, manganese, magnesium and iron. It is the abundances of these accessory elements that give garnets their physical properties of hardness and color. Iron rich garnets, for example, are in demand as abrasives because they are very hard and are used as sandpaper and abrasives. Garnets are also used in sandblasting, water-jet cutting equipment and as tumbling (polishing) media. The round nature of garnet crystals makes them useful as filter media.
The Shadow Mountain garnets have calcium substituting for iron; they are grayish, have many fractures and aren’t very durable. But their unusual size makes them pretty spectacular. They occur in the northwestern part of the northern Shadow Mountains. They form layers in the rocks alternating with limey siliceous (like limestone) layers. The accompanying picture shows a pen for scale.
Garnets often form from high alumina clay sediments, so the garnets at the Shadow Mountains tell us that there were sediments that alternated between clay and calcareous sandstone or siltstones. The episodes of metamorphism continued for a long time, and were subjected to plenty of heat and pressure to allow the garnets to grow large.
If you want to see the “golf ball” garnets for yourself, take the Shadow Mountain Road west from Highway 395 for about 7 miles. Follow the road through a 90 degree turn north for a mile. At the “intersection” turn right and head to the mountains. If you don’t have 4-wheel drive you might have to hike in for a few hundred feet. The garnets are in the ridges above the washes. Sometimes you can collect loose garnets that have weathered out of the rocks.