My taskbar has informed me that it's World Rock Day, so here's some cool rocks from Field Camp
Devil's Tower (Bear Lodge) is a gorgeous example of columnar jointing, but pictures never quite capture how large the individual columns are.
This guy here still has most of the hexagon shape, albeit a little squashed, and is easily 75cm/30in across and it's tiny compared to the larger columns that litter the foot of the tower (some were easily 5-6m/15-20ft). The rock itself is a wonderfully lichen encrusted porphyritic phonolite. Igneous and metamorphic petrology wasn't my strongest subject so I don't really remember what a phonolite is other than it's got an intermediate composition between felsic and mafic minerals. But porphyritic is a texture description meaning there are larger visible crystals (which cooled slowly) embedded in a matrix of tiny tiny crystals (which cooled quickly). Meaning the molten body that formed this monadnock had 2 separate cooling regimes, and thus why I subscribe to the idea that it's a volcanic neck.
The rocks at Clark's Fork, Wyoming preserve a stratigraphic column tipped almost perfectly on its side. At the left forming the tall grey cliffs are the Ordovician Bighorn and Carboniferous Madison Formations. The red stripe in the middle is the Triassic Chugwater Formation. And the TV static grey and white layers at the right is the Cretaceous Frontier Formation. The yellow ridge between the Chugwater and Frontier is the Cretaceous Cloverly Formation, the pink and white exposed hillside between the Chugwater and Cloverly is the famous Jurassic Morrison formation. And just barely visible as black patches on the hill between the Cloverly and Frontier is the Cretaceous Thermopolis Shale exposed as greasy paper thin shales and black smectites full of siderite nodules.
A fun lunchtime find, Oncolites! We were mapping thrust faults along the Beartooth Highway and my partner and I stopped to break for lunch along a low ridge formed by the Cambrian aged Meagher Limestone (pronounced Mahr apparently, no idea why there's so many extra letters). These odd structures are microbial sediment layers similar to stromatolites, but rather than standing tall and wave resistant they roll about in the tides and end up as little pellets.
Moving on to Utah, the red sandstone of the Entrada Formation created several features of note.
First and foremost, Arches! this is the very photogenic Delicate Arch in Arches National Park. The stock images of this arch don't really convey how huge it is, nor do they show that the walkway to is just the exposed rock face, which wouldn't normally concern me except that this angles down into a really steep bowl, don't slip! The presence of so many arches clustered within this one park is due to 2 geologic factors. 1 is the Entrada Sandstone is water permeable and sits atop the impermeable Carmel Formation. 2 is the Salt-Valley anticline. The anticline was caused by a collection of gypsum and salt in layers beneath what we see. Overburden pressure can cause evaporites like gypsum and salt to flow like a liquid, and here they collected and 'ballooned up' causing the rocks above to bend and fault vertically. Then at some point water dissolved the salt and gypsum and carried it away causing the dome to collapse, leaving lots of vertical faults, which would weather and fall and leave some of these slabs standing vertical. These slabs, made of the Entrada could absorb some water, but the water would 'get caught' at the bottom where it meets the Carmel, and then freeze and thaw and break the rock until a tiny hole had been carved at the bottom and wind could blow and hollow out the rest. And that's how the arches are formed here.
Over in Goblin Valley the Entrada weathers differently
These goblins are weathering features called hoodoos. Different rocks have different hardnesses, and here there is a layer of sandstone that's not quite so well cemented underneath a one that is. So wind eats away at the under-layer faster than the over-layer to make these mushroom shaped features!
Also this was the view stepping out of the shower house... that goblin is dummy thick
The Entrada also constitutes a major portion of the rocks in Little Wild Horse Canyon
Little Wild Horse and Bell canyons are slot canyons carved through a tilted block of Entrada, Navajo, Kayenta, and Wingate sandstones. These rocks were laid down by migrating dunes that covered a vast inland desert during the early to middle (and probably late) Jurassic.
And finally, a Granite pegmatite
Pegmatites are formed by the extremely slow cooling of a magma, allowing lots of time for minerals to coalesce and grow to massive sizes. Here my hammer (and Ocean Man's boot) serves as scale indicator for a massive book of muscovite (the shiny grey stuff) encased in orthoclase feldspar (the pink stuff).
Happy Rock Day!
