[Music] welcome to Blacktail Canyon in the Grand Canyon here in northern Arizona this is one of the probably hundreds maybe thousands of side Canyons that enter um the Colorado River this one's a real beautiful narrow Canyon that's been carved out by flash floods um over time and thanks for joining me geology Professor Shan Wily here on a trip through the Grand Canyon a rafting trip um and I wanted to stop in Blacktail Canyon because in addition to just the impressive scenery and Beauty here there is an astounding geologic feature one of the the Hallmark features of the Grand Canyon is exposed very nicely and is very accessible here in Blacktail Canyon and so the Canyon's been carved out by flash floods and as those flash floods have cut the canyon deeper they've cut progressively through the various layers of rock and here at ground level the main unit we have this very ledgy um kind of beige to buff colored unit is the toit sandstone a Cambrian age Sandstone um made out of grains of sand um some Pebbles as well a little bit bigger material on occasion and the depositional environment for the Topete Sandstone is a a beach a coastal setting where Rivers were feeding into uh the ocean and in places he might be able to pick out small crossbeds lots of layers as these River channels were flowing into into the ocean um but the big feature here is what lies just beneath the tapete sandstone and the tapete Sandstone is the lowest and oldest of the sequence of Paleozoic rocks here in the Grand Canyon it's a unit that's about 5 oh 30 or so million years old and in the Grand Canyon the toit Sandstone sits in most places directly on a much older layer known as the Vishnu shist and the Vishnu shist has been intruded by the zoraster granite and those two units the Vishnu shist and the zor W Granite are about 1.8 billion years old so what we have then is this huge J toos between the toit Sandstone the Cambrian age Coastal Sandstone sitting directly on these high-grade metamorphic rocks the Vishnu shist along with some intrusions of granite the zoraster granite and that contact between the two is what's known as an unconformity because there's a period of erosion or Rock layers missing between those two layers that should exist and that unconformity represents so much time that it's known is the great unconformity and here it is right here in front of you right here at ground level we have Vishnu shist so if I come over to this side we'll probably see um some of the vertical layering the fol iation in the shist um in this case it's been cut by a little bit of a quartz vein or a dyke there you can actually see some of the sparkly micro crystals in this hopefully there's a real nice one there and then sitting directly on top of that is the Topete sandstone and so this this contact right here represents the great unconformity so I'm literally touching 1.3 billion years of Earth's history between these two adjacent rock types the Sandstone above the metamorphic rock the high-grade metamorphic rock below so the real takeaway here and and the what I really want to convey is the um the real meaning of the great unconformity is not just the time although I think the time is really exceptional and um worth looking at but I think the bigger picture here for me is that these metamorphic rocks these basement rocks here represent a period of time when rocks were being created at Great depths below mountains tens of miles below the Earth's surface rocks are being squished heated compressed changed and altered into metamorphic rocks that's what forms the Vishnu shist here um along with intrusions of magma that then cooled and crystallized to form these granitic dkes that cut through the rock here um and and then a huge period of erosion right so these Coastal Sands are sitting on top of rocks that were formed deep within the the guts or the belly of a large mountain range and so this perfect contact here represents not just so much time but such a drastic change from one GE geologic setting to another one in which we're um we're forming mountains and then a huge amount of erosion and uplift probably uh to form the unconformity and we have to Plane off those mountains to such a degree that we can deposit the Sandstone on top of it um and so the the great unconformity here which is found in other places I've actually done uh two other videos one in Wyoming near Cody and one in Northern Utah um in the wasach mountains where we see it um but it's probably best known here in the Grand Canyon where you can see the great contact here here between the basement rocks in the Precambrian and the sandstones above sometimes along the contact we see it's a little bit more of a conglomerate with larger particles um which just indicates the erosion was a little more energetic so you're breaking up larger particles during that period of time um but just here's just a great view with these the foliation that the pressure here that formed these metamorphic rocks came from left to right from the sides rocks have been squeezed to form the the layers here and then you just have this perfectly truncated erosional surface uh kind of looking down the line and then the sandstones deposited on top of that so just a really exceptional view there of the gron Conformity uh and its Exquisite preservation here in Blacktail Canyon um we'll look at one more spot over here because it's just really so well exposed and and then we'll walk out to Canyon mouth here and maybe give you a quick view around the the actual Grand Canyon itself so nice little sand sand lens here within the base of the toit Sandstone some of the metamorphic rocks here actually looks like they're a little bit folded or the foliation is is bent a little here it's more vertical at the bottom here it's wrapping up and twisting a little bit and then just take you down to the mouth of the canyon here where we can see all the way up through these layers and we're not even looking up through the complete uh section here so what we have across the way here um if you can see is the tete sandstone and then the big Cliffs forming this but above us here that's the red wall limestone a Mississippi and aged Limestone um so there's still thousands of layers of rock above there that are out of view we've come far enough up the canyon in the canyon now and there that CR on Conformity surface undulates a little bit so now as we look from the canyon floor up we're completely within the toit Sandstone so let's just take you out to the river which you might be able to hear like all side Canyons um when these things have flash floods they push out a lot of rock and debris out into the Colorado River um and that creates Rapids so as the canyon or the river excuse me gets constricted by the flow of water it actually um causes the water to go a little faster and then all that Boulder and rock debris actually creates the the floor of the river Channel becomes um irregular and that's what creates some of the Big Rapids that we run when we do our white water rafting through here so take you up on this little Sandy burn here for a view of the great Colorado River at the heart of the Grand Canyon here looking Upstream The Ledges of Topete Sandstone red wall limestone a little bit of the supi group on top of that but and then this big debris fan here now swinging around we can actually see the rim of the canyon out in the distance there thousands of feet up and then looking back up into the mouth of Blacktail Canyon so thanks again for joining me on this uh little Excursion down here at the bottom of the Grand Canyon uh appreciate all the support you can provide donate button on the banner of my a YouTube channel there's a thanks button to the bottom right of the viewer um and then there's a PayPal Link in the video description But be sure to like share subscribe do all those things to help me promote GE ology and geology education hopefully you enjoyed this fun Excursion here from the Grand Canyon Blacktail Canyon