We took a day trip from Hanoi to Ninh Binh, the river area with the spectacular limestone hillocks that, as the tour guides say, is "Ha Long Bay on land". A couple of photos make the comparison clear:
That's the enclosed bay you get to via Luan cave, in Ha Long Bay. The next one is Ninh Binh:
I now know that this is called karst geology, and I think it's pretty remarkable that it all starts with an ancient build up of limestone up to 3 kilometres deep (!).
The rest of the geological history is explained here:
The recipe for this scenic splendor calls for just three simple ingredients: abundant limestone, vigorous recent tectonic activity, and a tropical climate. The first ingredient, a blanket of limestone up to 3 kilometers thick, accumulated during an extended, 130-million-year span from the Mid-Devonian through the Early Triassic, when the South China crustal block, to which Northern Vietnam belongs, was submerged beneath the ocean. The rest of Southeast Asia was appended to the South China block in a complex series of collisions with microcontinents during the Indosinian Orogeny, 250 million to 240 million years ago. The second ingredient — vigorous recent tectonic activity — began to roil the region about 40 million years ago, when Northern Vietnam again rumbled to tectonic life with the intrusion of granites and the onset of major faulting. The 1,000-kilometer-long Red River Fault is Northern Vietnam’s dominant tectonic feature. The Red River, which follows an exceptionally straight course through the country, passing through Hanoi en route, lent its name to the fault, which has been moving with right-lateral motion for the last 5 million years.....
..... movement along the Red River and companion faults has jumbled the surrounding blocks of marine limestone, tilting and folding them in complex ways and ultimately making them more susceptible to dissolution.
When bathed in groundwater, limestone easily dissolves, creating distinctive karst topography. Because this dissolution is speedy in Vietnam’s warm, tropical climate — the third and final ingredient needed to create Northern Vietnam’s legendary scenery — the area’s once-continuous blanket of limestone became riddled with caves until the subterranean texture resembled something like Swiss cheese. The cave roofs eventually collapsed, creating sinkholes, and ultimately, the bedrock left undissolved and still standing comprised landscapes of limestone spires. Northern Vietnam and southeastern China constitute the world’s largest (and, many would argue, the most spectacular) karst region, which visitors can easily access at several stunning locations.
As any survey of travel vlogs on Youtube will show, both Ninh Binh and Ha Long Bay are incredibly popular, with local and international tourists. There seem to be at least two different places where you can start on a 2 hour or so rowboat trip on the river around Ninh Binh, and I don't even know the name of the place where we started, but it had a new looking, very large tourist building:
The scenery on the river is, of course, spectacular, and there are parts with some type of temple on them which it seemed some people got to visit, but not us. (I suspect they are modern, and part of the intense development of the area into a prime tourist spot.) There are long caves you go through (on boat), too.
There's also a famous lookout in the area, but I didn't go up it due to my wife's current hip problem. We rode some bicycles instead. The fields were currently a bit rice bare, but I can imagine this ride looking good when the rice is fully grown:
And unfortunately, when I see ducks in this setting, my mind does wander to the question of whether they are breeding the next dangerous bout of avian flu:
Being a bit science nerdy, the main thing I would have liked to have added somewhere on the tour is an explanation of the geology of the area.
But yeah, like Ha Long Bay, it's an area you just have to visit if in Hanoi and the weather is up to it.
Back to work now...
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