At first there is only sand
The Costești sand was laid down early in the Maeotian, a geological age that began about 7.6 million years ago. Then it was covered by other layers. Between its grains are tiny gaps full of water.
The trovants of Costești, Romania
Do the stones of Costești really grow after rain?
Beside the road from Râmnicu Vâlcea to Horezu, in southern Romania, sit round stones called trovants, some as tall as a person. The internet says they are alive: that they grow, multiply and swell after rain.
They do not grow. Trovants formed underground, while the sand was buried and soaked with water. What changes after rain is the hill: water carries off the loose sand, and the harder stone stays behind and shows more and more.
The sand at Costești was laid down early in the Maeotian, an age that began about 7.6 million years ago. How fast it hardened into stones nobody has measured. Similar stones in a Scottish sandstone took up to a few million years.
Make a trovant#
The drawing below is a slice through the hill, 4.5 metres wide. Move time with the slider, or read on and the drawing follows the text.
Loose sandWidth of the big stone: —How much of it shows: —
Something in the sand gathers the cement
In concretions generally, the cement settles around a core: a piece of shell, a leaf, a tooth. Geologists call it a nucleus. In the drawing, the nuclei are the black dots.
Touch the sand to drop your own seed, then move time forward.
Water glues the grains together
Water seeping through the sand carries dissolved calcite, the mineral limestone is made of. Around the nucleus, the calcite settles between the grains and glues them, from the centre outwards. The reserve's management plan calls this centrifugal growth. Some stones end up ringed inside, others solid: the plan says their structure can be massive or concentric.
Where the cement arrives equally from every side, the stone comes out round. Where it travels more easily sideways, along the layer, the stone comes out flattened.
Two stones that touch become one
In the drawing, when two nuclei are close, their stones grow towards each other and join. That is one way a trovant could get its “babies”: one large body with bulges on it. The plan says only that uneven cementation of the sand gives trovants their varied, sometimes bizarre shapes.
The loose sand is carried off; the stone stays
Much later, water and people start to carry the hill away. Unglued sand goes easily; the harder stones stay where they are or roll to the foot of the bank.
At Costești the quarry wall is 30 metres high. The management plan says its upper edge keeps moving because of erosion and the sand dug from its base, and the digging brings more trovants to the surface.
Now make it rain
The figure on the right is as tall as you set with the slider in “How big are they?”. Look at the two numbers under the drawing before you answer.
Just as wide. The rain took sand from around it, so more of the stone now shows. Anyone passing after a storm sees a “bigger” stone, though the stone is the same. Buzău County Council puts it plainly, of the county's trovants: wind and rain clean away the soft sand.
In the drawing one storm carries off 5 centimetres of sand, so that something visibly happens. How much a real storm removes at Costești has not been measured, as far as we know.
How fast does a trovant grow?#
Many websites say trovants grow 4–5 centimetres every 1,200 years. Before you read on, guess.
Nobody, as far as we could find. The number appears on the Romanian Wikipedia with no source, in the same paragraph that calls the name “growing stones” wrong. One blog gives it as “4–5 cm in about 1,000 years”.
The reserve's management plan, written by specialists, gives no rate at all. It says only that trovants formed in early diagenesis, soon after the sand was buried.
How many years a round stone takes to grow, according to five sources
“4–5 cm every 1,200 years” the internet claim, no source; a stone 1 m across
12,000–15,000 years
Model for a Scottish sandstone Wilkinson and Dampier, 1990, as cited by Paxton and colleagues; a stone 1 m across
4 million years; over 6 if the water is still
Real concretions in the same sandstone Paxton and colleagues, 2023; from the age of the lavas above them
2.6 million years at most
Concretions in clay, Japan calculated from Yoshida and colleagues, 2018; 1 m
from about two months to 158 years
Moeraki boulders, New Zealand after Wikipedia, citing Boles and colleagues, 1985; 2 m
4–5.5 million years
The closest comparison is a sandstone on the Isle of Skye, in Scotland, with calcite concretions like the trovants. In 1990 the geologists Wilkinson and Dampier calculated that such a stone, a metre across, would have needed about 4 million years if water flowed through the sand and more than 6 million if it stood still. In 2023 a team led by R. B. Paxton used published dates of the lavas above them and showed the real stones formed in 2.6 million years at most, roughly twice as fast as the fastest calculation.
Had trovants grown 4–5 centimetres every 1,200 years, a stone a metre across would have formed in 12,000–15,000 years: a few hundred times faster than the calculation for the Scottish sandstone.
Other concretions grow far faster. Hidekazu Yoshida and his colleagues studied round stones in marine clays in Japan, formed around decaying organic matter, and calculated that their edge advances by a hundred-millionth to a hundred-thousandth of a centimetre a second. At those speeds a stone a metre across would form in a few hundred years at most. Earlier estimates gave times at least a thousand times longer. So the speed depends on the rock, on the water and on what it carries. For trovants, nobody has published a measurement.
How big are they?#
From a few millimetres to a few metres, the sources say. Set the slider to your height.
At Ulmet, the same council says, trovants can reach several metres high. Wikipedia says some at Costești weigh “several tonnes”, without a source. Nobody gives the measured mass or width of a particular stone, so no “largest” is named here.
Who put the stones there?#
The Trovant Museum is a 1.1-hectare nature reserve, protected by Government Decision 1,581 of 2005. It lies beside the DN 67 road at the entrance to Costești, at the foot of a sand quarry.
Many of the stones tourists photograph were set out there by people. In 1996 a team from the Faculty of Geology and Geophysics in Bucharest laid the site out as an open-air museum and arranged the trovants at the foot of the quarry, with signs beside them. Since then, the management plan says, some stones have been taken, others moved, and quarrying has brought others to the surface.
The reserve is a protected area, and its plan forbids uses that harm it. Some stones have already been taken, the same plan says. A stone taken home is a stone nobody else will see.
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Where are trovants found?#
Costești is the best-known site but not the only one. The reserve's plan says such stones occur elsewhere in Romania; Costești has a great many in a small space.
- Costești (Vâlcea)
- Oteșani (Vâlcea)
- Ulmet (Buzău)
- Chiojdu (Buzău)
- Feleacu (Cluj)
- Cașolț (Sibiu)
- Hășdat (Hunedoara)
- Cornii de Sus (Bacău)
- Domnești (Bistrița-Năsăud)
- Cliț (Sălaj)
At Oteșani, some 16 kilometres from Horezu, trovants come out of the bank of the Gresarea stream, dug out by the torrents after rain. It is the same as in the drawing above: water brings the stone into view; it does not make it bigger.
Where does the word come from?#
“Trovant” is a word of Romanian geologists. Gheorghe Munteanu Murgoci introduced it in “The Tertiary of Oltenia”, in 1907.
The dictionary gives it an Italian origin: “trovante”. Italian geologists use the word for an erratic boulder, a stone that has ended up far from where it formed; Italian took it from Friulian, and it may be related to an old French word for “vagabond”. Romania's trovants formed in the very sand they come out of.
Elsewhere, geologists call stones of this kind concretions. Some hold a fossil at their centre, which is why fossil hunters break them open.
True or false?#
Eight statements about trovants. Choose, then see what the sources say.
After heavy rain, a trovant is bigger than it was before.
Rain washes away the loose sand around the stone, so more of it shows. The stone itself formed underground long ago, says the reserve's management plan.
A trovant is sand glued together by calcite, the mineral limestone is made of.
The Romanian dictionary defines it as part of a sand bed “cemented by calcareous waters”.
The growth of “4–5 cm every 1,200 years” was measured by geologists.
As far as any source shows, the number sits unsourced on the Romanian Wikipedia; one blog gives it as “4–5 cm in about 1,000 years”.
The stones in the Trovant Museum lie exactly where nature left them.
The museum was laid out in 1996 by a team from the Bucharest Faculty of Geology, who arranged the trovants at the foot of the quarry. Some have since been taken and others moved.
The word “trovant” came into Romanian from Italian.
The dictionary gives its origin as the Italian “trovante”.
The name “trovant” was introduced by a Romanian geologist in the early 20th century.
Gheorghe Munteanu Murgoci used it in “The Tertiary of Oltenia”, 1907.
Trovants are found only at Costești.
They occur elsewhere in Romania too, from Ulmet in Buzău to Feleacu near Cluj. Costești simply has a great many in a small space.
Similar round stones, also of cemented sediment, exist in other countries.
The Moeraki boulders, up to 2.2 metres across, lie on a beach in New Zealand, and small iron-oxide balls weather out of the sandstone of Utah.
Answer all eight to see your score.
Sources and what is not known#
The facts about the geology at Costești, the museum and the reserve come from the reserve's management plan, an official document. The definition and origin of the word come from dictionaries. What a concretion's nucleus is, the Moeraki figures and the list of localities come from Wikipedia. For growth speeds and comparisons I used scientific papers on other concretions, saying each time which rock they describe.
The drawing at the top is schematic: it follows the steps the plan describes, but the shapes, the sizes and how much a storm removes are chosen so the process can be seen.
What the sources do not say
- How fast any trovant grew. There is no published measurement on trovants that we could find.
- How old the trovants are. The Costești sand is Maeotian; when the cement hardened has not been dated. For the Buzău trovants the county council writes that the sand hardened “probably in the last few hundred thousand years”, without saying how it knows.
- How big the largest trovant is. “Several tonnes” and “4.5 metres across” circulate without a source.
- How much sand a storm removes at Costești. The plan says only that the quarry edge moves.
Documents and dictionaries
- Management plan of the Muzeul Trovanților nature reserve, approved by Order no. 919/2014 (in Romanian). legeaz.net Geology, the age of the sand, the museum's history, the reserve's boundaries and area.
- Romanian explanatory dictionary (DEX) and dictionary of neologisms (MDN), s.v. “trovant” (in Romanian). dexonline.ro The definition and the Italian origin of the word.
- Vocabolario Treccani, s.v. “trovante” (in Italian). treccani.it The Italian sense: a synonym for erratic boulder.
- Buzău County Council, “Sites with trovants in Buzău County” (in Romanian). cjbuzau.ro The trovants of Ulmet and Chiojdu, their sizes and the part rain plays.
Scientific papers
- Palcu, D. V., Vasiliev, I., Stoica, M. and Krijgsman, W. (2019). The end of the Great Khersonian Drying of Eurasia: magnetostratigraphic dating of the Maeotian transgression in the Eastern Paratethys. Basin Research 31, 33–58. doi.org When the Maeotian began and ended.
- Paxton, R. B., Andrews, J. E., Dennis, P. F., Marca, A. D. and Holmden, C. (2023). Concretionary cementation of a Scottish Middle Jurassic sandstone by hot, Paleocene fluids: a clumped isotope study. Journal of the Geological Society. doi.org How long the Skye sandstone concretions took; the Wilkinson and Dampier (1990) calculation, cited here.
- Yoshida, H. et al. (2018). Generalized conditions of spherical carbonate concretion formation around decaying organic matter in early diagenesis. Scientific Reports 8, 6308. nature.com Growth rates of spherical concretions in marine clays in Japan.
- Chan, M. A. et al. (2004). A possible terrestrial analogue for haematite concretions on Mars. Nature 429, 731–734 (abstract). nature.com What a spherical concretion needs: permeable rock, flowing groundwater and a reaction front.
- Zalewska, N. et al. (2025). Abstract EPSC-DPS2025-1125, Europlanet Science Congress. meetingorganizer.copernicus.org The trovants' cement, calcite between quartz grains; “up to 4.5 metres”, unsourced.
Supporting sources
- Romanian Wikipedia: “Trovant” and “Muzeul Trovanților”. ro.wikipedia.org The list of localities and the coordinates; also where the unsourced “4–5 cm in 1,200 years” appears.
- English Wikipedia: “Moeraki Boulders” and “Concretion”. en.wikipedia.org The size and age of the Moeraki boulders, after Boles and colleagues (1985); what a concretion's nucleus can be.
- Natural Earth, 1:10,000,000 country boundaries (public domain). naturalearthdata.com The outline of Romania on the map.