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Geology, Palaeontology & Archaeology / Re: Angular Sand?
« on: 06/05/2011 00:57:59 »First, I am assuming that the Sugar Loaf Mt you are referring to is close to US Highway 1 northwest of Florence, SC. There is a Sugar Land Mountain in the Thrust Section of the interior Piedmont that is a lot more interesting.
The one in SC is what I was referring to.
The more angular a sand grain the YOUNGER it is. These sand grains had just eroded from fine grained granite or diorite (probably - possibly could have been some other rock type.) These sand grains just came from a fine to very fine grained igneous source. So they didn't get transported very far or they would have been more rounded.
I understand that. It's why I was wondering. The sand all around is much smaller and is rounded. The sand in the rock is massive and very sharp.
Secondly, the flatness of the formation or geologic unit under it doesn't really have much to do with the formation over it or the sand grain unit itself. The were laid down sequentailly, one on top of each other.
Is that true for a dune field? From what I have read, the Sandhills are supposed to be the result of wind blown sand. The Ocean level fell and exposed the coastal plain and the sand then blew into dunes and traveled northwest until vegetation stopped their progress. I know the sands of the coastal plain are the result of river effluent. But the sands of the Sandhills are supposed to have been blown there. How can angular sand be blown anywhere and then cemented together at the top of a dune?
Since the geologic map of S. Carolina shows the area of the park to be in the Cretaceous, the area we are talking about was having very rapid erosion and deposition from the west. There were large deltas spewing sand all over the place out in front them that were then much higher than the Appalachian Mountains are now. The layered water-placed deposits were covered up as fast as they were put down.
So there was a 4000ft cliff face at the ocean? I had no idea the Fall Line was that pronounced at any point. I have read the ocean was 800ft higher than it is today, but not that the very edge of the piedmont was so tall. Well, I did read that it was at one time, but when Africa broke away didn't it pull the piedmont and thin it out and that was when the mountains that were here went away? Didn't Africa leave 200 million years ago? The cretaceous was 65 million years ago. Weren't the mountains long gone by then? I had been given the impression that the topography of today's piedmont is not much different than it was 65 million years ago other than the ocean being at the Fall line and the hills being a little more pronounced than they are today.
I think my timeline is skewed or I have read the wrong articles about geology.
As the depositional center moved from one area to another, as the mouth of the Mississippi is now try to do by pouring its water out through the Atchafalaya River Basin, Erosion takes place and the sands are transported along shore, sorted in size, and re-deposited into sand dunes. These then get blown in-land as dunes in specific areas. They get covered with mud and become the perfect place to allow water to flow through. As the iron rich water flowing through the sand gets closer to the sea, the amount of iron it can carry diminishes, the iron is deposited on the sand grains and the grains are eventually cemented together.
Let me see if I got my head around this correctly. 65 million years ago, the mountains of the piedmont were like the Appalachians are today. There were short rivers running into the sea. These rivers deposited sand where the Sandhills are today. The fossil dunes are not the remnants of wind blown dunes, but water formed dunes under the ocean in a continental shelf type environment. As the water receded the rivers cut new channels and eventually there was an oxbow lake or something of the sort and bacteria managed to stay there long enough for it's secretions to cement the sand together into sandstone. The areas that look like folded rusted metal are bacteria mats that have sunk and collapsed on the bottom and then were covered in sand. The sand washed away and the river dried up or moved two miles west and then erosion took away all the surrounding sands and left this "fossil oxbow" sitting high and dry.
Is this right? How did the piedmont sink so far so fast? I thought the crust was under compression in this area since we are locked into the spreading ridge in the Mid Atlantic. How would it sink? I thought also that the land slowly rose as erosion removed the overburden. How did it rise and fall at the same time?
Why do they say the Sandhills are the result of wind and not water?
Reverse the process of erosion 50 million or more years later and you get Sugar Loaf Mountain.
REMEMBER: Sand is very hard, It takes a very long time to go from an angular grain to a rounded grain. Usually it takes 4 to 8 erosional cycles for the round grains to be formed