Welcome to Blacktail Canyon in the Grand Canyon here in northern Arizona. Welcome to Blacktail Canyon in the Grand Canyon Here in northern Arizona, this is one of the probably hundreds, This is one of the probably hundreds, maybe thousands of side canyons that enter the Colorado River. maybe thousands of side canyons that enter the Colorado River. This one's a real beautiful, This one's a real beautiful, narrow... narrow canyon that's been carved out by flash floods over time.
canyon that's been carved out by flash floods over time. And thanks for joining me, And thanks for joining me, geology professor Sean Bilsey here on a geology professor Sean Bilsey here on a trip through the Grand Canyon, trip through the Grand Canyon, a rafting trip. a rafting trip, and I wanted to stop in Blacktail Canyon because in addition to just the impressive scenery and beauty here, 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. There is an astounding geologic feature. One of the hallmark features of the Grand Canyon is exposed very nicely and is very accessible here in Blacktail Canyon. One of the hallmark features of the Grand Canyon is exposed very nicely and is very accessible here in Blacktail Canyon. And so the canyon has been carved out by flash floods, And so the canyon has been carved out by flash floods, and as those flash floods have cut the canyon deeper, and as those flash floods have cut the canyon deeper, they've cut progressively through the various layers of rock.
they've cut progressively through the various layers of rock. And here at... And here at ground level, Ground level, the main unit we have, the main unit we have, this very ledgy, this very ledgy, kind of beige to buff colored unit is the Tapeats sandstone, kind of beige to buff-colored unit, is the Tapeat Sandstone, a Cambrian age sandstone. a Cambrian-age sandstone. made out of grains of sand, made out of grains of sand, some pebbles as well, some pebbles as well, a little bit bigger material on occasion.
a little bit bigger material on occasion. And the depositional environment for the Tapeats Sandstone is a beach, And the depositional environment for the Tapeats Sandstone is a beach, a coastal setting, a coastal setting, where rivers were feeding into the ocean. where rivers were feeding into the ocean. And in places, And in places, you might be able to pick out small crossbeds, you might be able to pick out small crossbeds, lots of layers, lots of layers, as these river channels were flowing into the ocean.
as these river channels were flowing into the ocean. But the big feature here is what lies just beneath the Tapeats Sandstone. But the big feature here is what lies just beneath the Tapeats Sandstone. And the Tapeats Sandstone is the lowest and oldest of the sequence of Paleozoic rocks here in the Grand Canyon.
And the Tapeats Sandstone is the lowest and oldest of the sequence of Paleozoic rocks here in the Grand Canyon. It's a unit that's about 500 and, It's a unit that's about 500 and... Oh, oh, 30 or so million years old.
30 or so million years old. And in the Grand Canyon, And in the Grand Canyon, the Tapeats Sandstone sits, the Tapeats Sandstone... sits in most places directly on a much older layer known as the Vishnu Schist. in most places, directly on a much older layer known as... the Vishnu Schist.
And the Vishnu Schist has been intruded by the Zoroaster Granite. And the Vishnu Schist has been intruded by the Zoroaster Granite. And those two units, And those two units, the Vishnu Schist and the Zoroaster Granite, the Vishnu Schist and the Zoroaster Granite, are about 1.8 billion years old.
are about 1.8 billion years old. So So what we have then is this huge juxtaposition between So what we have then is this huge juxtaposition between the Tapeats sandstone, The Tapeat sandstone, the Cambrian Age coastal sandstone, the Cambrian Age coastal sandstone sitting directly on these high-grade metamorphic rocks, sitting directly on these high-grade metamorphic rocks, the Vishnu Schist, the Vishnu Schist along with some intrusions of granite, along with some intrusions of granite, the Zoraster granite. the Zoroaster granite. And that contact between the two is what's known as an unconformity, And that contact between the two is what's known as an unconformity because there's a period of erosion. because there's a period of erosion or rock layers missing between those two layers that should exist.
or rock layers missing between those two layers that should exist. And that unconformity represents so much time that it's known as the great unconformity. And that unconformity represents so much time that it's known as the great unconformity.
And here it is right here in front of you, And here it is right here in front of you, right here at ground level. right here at ground level. We have Vishnu's schist. We have Vishnu's schist.
So if I come over to this side, So if I come over to this side, we'll probably see some of the vertical layering, we'll probably see some of the vertical layering, the foliation in the schist. the foliation in the schist. In this case it's been cut by a little bit of a quartz vein or a dike there.
In this case, it's been cut by a... little bit of a quartz vein or a dike there. You can actually see some of the sparkly microcrystals in this hopefully. You can actually see some of the sparkly microcrystals in this, hopefully.
There's a real nice one there. There's a real nice one there. And then sitting directly on top of that is the tapit. And then sitting directly on top of that is the Tapeats Sandstone. sandstone.
And so this contact right here represents the Great Unconformity. And so this contact right here represents the Great Unconformity. So I'm literally touching So I'm literally touching 1.3 billion years of Earth's history between these two adjacent rock types. 1.3 billion years of Earth's history between these two adjacent rock types, the sandstone above, above the metamorphic rock, the metamorphosis.
the high-grade metamorphic rock below. the high-grade metamorphic rock below. So the real takeaway here and what I really want to convey is the real meaning of the great unconformity is not just the time, So the real takeaway here, and what I really want to convey is the real meaning of the great unconformity is not just the time, although I think the time is really exceptional and worth looking at, although I think the time is really exceptional and worth looking at, but I think the bigger picture here for me is that these metamorphic rocks, 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, 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. tens of miles below the earth's surface. Rocks were being squished, Rocks were being squished, heated, heated, compressed, compressed, changed and altered into metamorphic rocks.
changed, and altered into metamorphic rocks, And that's what forms the Vishnu Schist here, and that's what forms the Vishnu Schist here, along with intrusions of magma that then cooled and crystallized to form these granitic dikes that cut through the rock here. along with intrusions of magma that then cooled and crystallized to form these granitic dikes that cut through the rock here. And then...
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 geologic geologic setting to another one in which we're um then a huge period of erosion, right? So these coastal sands are sitting on top of rocks that were formed deep within 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 geologic setting to another, one in which we're forming mountains, We're forming mountains and then a huge amount of erosion and uplift probably 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. and then a huge amount of erosion and uplift probably to form the... the unconformity and we have to plane off those mountains to such a degree that we can deposit the sandstone on top of it.
And so the great unconformity here, And so the great unconformity here, which is found in other places. which is found in other places, I've actually done... I've actually done two other videos, two other videos, one in Wyoming near Cody and one in northern Utah in the Wasatch Mountains where we see it.
one in Wyoming near Cody and one in northern Utah in the Wasatch Mountains where we see it. But it's probably best known here in the Grand Canyon where you can see the great contact here between the basement rocks and the Precambrian and the sandstones above. But it's probably best known here in the Grand Canyon where you can see the great contact.
here between the basement rocks and the Precambrian and the sandstones above. Sometimes along the contact we see it's a little bit more of a conglomerate with larger particles which just indicates the erosion was a little more energetic so you're breaking up larger particles during that period of time 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 side Sometimes along the contact we see it's a little bit more of a conglomerate with larger particles which just indicates... the erosion was a little more energetic so you're breaking up larger particles during that period of time 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 these rocks have been squeezed to form these the layers here and These rocks have been squeezed to form the layers here.
And then you just have this perfectly truncated erosional surface kind of looking down the line. then you just have this perfectly truncated erosional surface Kind of looking down the line and then the sandstones deposited on top of that. And then the sandstone's deposited on top of that.
So just a really exceptional view there of the Great Unconformity and its exquisite preservation here in Blacktail Canyon. So just a really exceptional view there of the Great Unconformity and its exquisite preservation here in Blacktail Canyon. We'll look at one more spot over here because it's just really so well exposed. We'll look at one more spot over here because it's just really so well exposed and then we'll walk out the canyon mouth here and maybe give you a quick view around the actual Grand Canyon itself. And then we'll walk out the canyon mouth here and maybe give you a quick view around the actual Grand Canyon itself.
itself. So nice little sand lens here within the base of the Tapeats sandstone. So nice little sand, sand lens here within the base of the peat sandstone. Some of the metamorphic rocks here actually looks like they're a little bit folded or the foliation is bent a little here.
Some of the metamorphic rocks here actually looks like they're a little bit folded or the foliation is bent a little here. It's more vertical at the bottom here. It's more vertical at the bottom here. It's wrapping up and twisting a little bit.
It's wrapping up and twisting a little bit. And then just take you down to the mouth of the canyon here where you can see. Then just take you down to the mouth of the canyon here where you can see All the way up through these layers, All the way up through these layers, and we're not even looking up through the complete section here. and we're not even looking up through the complete section here. So what we have across the way here, So what we have across the way here, if you can see, as you can see, is the Tapeat Sandstone, is the Tapeat Sandstone, and then the big cliffs forming this butte.
and then the big cliffs forming this butte. above us here, above us here, that's the Redwall limestone, that's the Redwall limestone, a Mississippian-aged limestone. a Mississippian-aged limestone. So there's still thousands of layers of rock above there that are out of view. So there's still thousands of layers of rock above there that are out of view.
But we've come far enough up the canyon, We've come far enough up the canyon, in the canyon now, in the canyon now, and there and that that Crate-on-Conformity surface undulates a little bit. Crate-on-Conformity surface undulates a little bit. So now as we look from the canyon floor up, So now as we look from the canyon floor up, we're completely within the Tapeats Sandstone.
we're completely within the Tapeats Sandstone. So let's just take you out to the river, So let's just take you out to the river, which you might be able to hear. which you might be able to hear. Like all side canyons, Like all side canyons, when these things have flash floods, when these things have flash floods, they push out a lot of rock and debris out into the Colorado River, they push out a lot of rock and debris out into the Colorado River, and that creates rapids. and that creates rapids.
And so as the canyon or the river, So as the canyon or the river... excuse me, me gets constricted by the flow of water it actually causes water to go a little faster and then all that boulder and rock debris actually creates the the floor of the gets constructed, constricted by the flow of water. It actually causes the water to go a little faster and then all that boulder and rock debris actually creates the floor of the river channel becomes irregular the river channel becomes irregular, and that's what creates some of the big rapids that we run when we do our whitewater rafting through here. and that's what creates some of the big rapids that we run when we do our whitewater rafting through here. So I'll take you up on this little sandy berm here for a view of the great Colorado River.
So take you up on this little sandy berm here for a view of the great Colorado River. at the heart of the Grand Canyon here. At the heart of the Grand Canyon here, looking upstream, Looking upstream, the ledges of Tapeats Sandstone, the ledges of Tapeats Sandstone, Redwall Limestone, Redwall Limestone, a little bit of the Supai Group on top of Bat Butte, a little bit of the Supai Group on top of that butte. And then this big debris fan here now swinging around.
and then this big debris fan here now swinging around. We can actually see the rim of the canyon out in the distance there, We can actually see the rim of the canyon out in the distance there, thousands of feet up, thousands of feet up, and then looking back up into the mouth of Blacktail Canyon. and then looking back up into the mouth of Blacktail Canyon. Thanks again for joining me on this little excursion down here at the bottom of the Grand Canyon. Thanks again for joining me on this little excursion down here at the bottom of the Grand Canyon.
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