This is the best thing is going sideways because then if you fall off, it's not the end of the world. Some of this is actually pretty hard. Small holds and a little balance. I have the Laramide orogeny to thank for these rocks, not the rocks themselves, of course, this is the Fountain Formation and it's 300 million years old. The age of the Ancestral Rockies. I'm climbing on the underside of a flat iron. These holds are basically bedding planes that have eroded a little bit and where they've eroded, they got great handholds inside where a little bit rock has eroded away. There's a big one. But the rocks themselves were originally horizontal and they've been tilted into this position by the younger mountain building phase called the Laramide between about 70 and 40 million years ago, around the end of dinosaur time, which of course, we track to about 65 million years, the end of the Mesozoic. These are late Mesozoic, early Cenozoic uplifts that tilted the rocks. These are great handholds. One of my favorite little climbs can get pretty high but solid rock , hop over here. I get on top of the flat iron, low angle face on the other side and the flat planes that go for 1,000 miles. I'm here with one of these miniature flat irons, one of these tilted slabs of Fountain Formation. The idea that we're trying to really think about right now is that these rocks were originally flat line. They were originally deposited as they eroded out of the ancient ancestral Rockies. The Fountain Formation was more or less flat line sediments, layer after layer after layer. It's this more recent period of mountain uplift, uplift being orogeny and geological terms called the Laramide orogeny, that tilts these rocks to their upwards of 60 degree incline and they become flat irons. The flat iron rock itself is dominantly sandstones and conglomerates, they're very resistant to erosion. But also we find near the margins of those rivers and streams crumblier rock. This stuff here, which we would call shale, is just finer grained sediment. But if we keep coming over here, there's something really unusual. This is not a sedimentary rock, this is an igneous rock. This is a rock that has an age of right around the time of flat iron tilting. This is an intrusion of magma that squirted into the Earth about 65 million years ago. What is this and what's it doing here? Bolder sits more or less on the north eastern extension of a whole series of 70-45 million year-old igneous intrusions that squirted up into the crust of Colorado right around the time of Laramide moutain buidling, Laramide orogeny. This is an extension of what we call the Colorado mineral belt magmas. We don't know for sure, but one of the puzzles of the Fountain Formation is that it's really hard, it's really solid. It makes for a great rock climbing. The flat irons are really solid, really hard rock. The flat iron rock has a really special cementation, particularly in the area of boulder and that's why boulder has these beautiful flat irons, both the little ones and the big ones behind the city that are so iconic. The cementation, the thing that's holding the grains together in these rocks is a feldspar called agile area that precipitates out of fluids. Relatively low to moderate temperature fluids and we think one possibility is that magma intrusion helps circulate the fluids in the crust that ultimately gave this agile area cement to the Fountain Formation and the resistant flat iron rock. Let's look at this igneous intrusion here. It's got some little teeny tiny black particles in here. Those are biotite. There's some bright reflectant pieces, that's a different feldspar called sanidine. But most interestingly, what do we see on the face here? Some little black particles that are some of the flat iron or Fountain Formation shale that seem to be included in this rock. Come on over here and let's take an even closer look. Here's the igneous intrusion. This was a molten rock at about 65 million years ago. Look how it has some pieces of the shale included in it. There is a piece, there is a piece, there, there, there, here, here, here. These are all little chips of shale that are actually included in the intrusion, in the magma. We call these included fragments and we use one of Steno's principles that we learned about in one of the lectures, the principle of inclusion. If a fragment of included rock is here, it must be older than this and that makes sense. This is 65 million year-old intrusion. These are 300 million-year old shales. We think these were here into the flat iron rock was squirted this intrusion at about 65 million and we've got pieces of the adjoining flat iron shale included in the magma. All through Colorado between about 70 and 40 million years ago, lots of squirts of magma, lots of intrusions coming through associated with things like the Colorado mineral belt. Unlike the Ancestral Rockies at 300 million years ago, with no magma, the mountains were uplifting, but no squirts of magma. It's interesting we think that the Laramide orogeny is linked to a subducted slab traveling in a flat slab mode underneath North America all the way from the western margin, what now is California, Oregon, and Washington. This slab of oceanic lithosphere traveling underneath North America and that's the initiator of much of this magma that we see during Laramide time, and this is just one example of it. Other bits and blobs of magma coming up in Colorado to form the Colorado mineral belt and associated fluids, high temperature water that generates gold, molybdenum, silver, all the precious metals that got mined particularly in the late 19th century here in Colorado.
