Geological Trail Intro Part 3

Almo Pluton 

Most of the pinnacles at City of Rocks and Castle Rocks are made of granite from the Almo pluton. The pluton started out as an intrusive body of magma and has a convoluted contact with the Green Creek Complex. It is the youngest rock at City of Rocks and formed about 28 million years ago. 

Unlike the granite of the Green Creek Complex, the Almo pluton has an equigranular texture. Notice in the photograph the small uniform size of the crystals in the Almo pluton in contrast to the variety of sizes in the Green Creek Complex. 

Both types of granite are cut in many places by dikes of pegmatite, a very coarse-grained granitic rock that usually forms during the last stages of crystallization of a large granitic pluton. The pegmatites at City of Rocks contain large quartz and feldspar crystals about the size of a fist.