Imotski is south-east of Omis and this drive inland from Omis to Imotski takes about an hour. The surrounding landscapes encapsulate many of the karst features that have been associated with Croatia in previous posts.
The Imotski road trip leaves Omis and quickly climbs to the uplifted plateau behind the town. A narrow but more than adequate road passes through a number of small limestone villages surrounded by their adaptive agricultural land use. The road intersects the E65 Motorway as a signpost for the remainder of the drive across the Croatian karst to Imotski. Two of the world’s largest sinkholes wait to be explored. The remainder of the day can be spent seeking out Imotski’s other attractions and then wrestling with choosing the next route to get to Dubrovnik. That is the next post.
The drive from Omis to Imotski ends at the enormous Imotski sinkholes. The journey also provides a glimpse of Croatia’s limestone interior and land use. The sinkholes at Imotski surprise by both their width and depth, and reinforce our understanding of the undermining of karst
Karst is a landscape composed of carbonate rock such as limestone, dolomite and gypsum. Each of these varieties are susceptible to dissolution in acidic water. This leads to the recognisable karst features such as grey, craggy terrain, sinkholes, underground streams and caves.
What to See on the Drive to Imotski
The term applied generally to rocks containing calcium carbonate, such as limestone and dolomite
Once on top of the fault scarp the D70 winds its way along the plateau through small towns such as Ostrvica, Smolonge and Zvecenje. These towns demonstrate the region’s adoption of limestone as a building material.
An interesting but often unrecognised feature is the structure and variation of the soil over the karst. These variations impact on the land use and crops that can be grown. Small valleys and basins that have formed in the Paleogene limestone either as a result of tectonics or erosion provide space for agricultural land use. They can act as shallow catchments for water that drains off the slopes and which may carry humus or other organics.
The soils through Ostrvica are very shallow with minimal development. They are undermined by continuous hard or fragmented rock which makes up to 80% of the profile. However, although shallow, it is humus rich.
Intersection with the Tollway Towards Imotski
The D70 intersects with the E65, a toll road. Once on the E65, look for the D39 into Sestanovac on the left. You can continue straight ahead as shown on the Google map but our choice was to follow the D39.
When driving along the D39 the landscape becomes influenced by soils that in some areas are in the early stages of formation. Insufficient time has occurred for the primary minerals to weather and form separate layers of accumulated clay. However, a reddish colour of the soil indicates clay has formed under the surface.
The D39 continues until it intersects with the D60 beyond which a right turn signals the final part of the drive to Imotski.
What the Karst Looks Like and Why
Along the entire drive to Imotski you will observe features associated with the karst landscape. So what to look for and what are their origins.
The greyish karst terrain along the drive is invariably sparse and craggy. The calcareous surface has over the millennia been subjected to weathering and erosion. Consequently, the rock you will see across the terrain has become heavily pitted, hollowed out and angular. It is also very apparent that surface water in the form of lakes and streams which are strongly associated with other landscapes are nowhere to be seen. The sometimes barren, rocky ground is devoid of surface water because the karst is extremely porous.
Water, irrespective of its source, forms cylindrical hollows across the terrain by the dissolution of calcareous minerals. The mineralised water penetrates to lower depths and will usually be captured by fissures or sinkholes and become part of subterranean streams and rivers. These form a network of pathways and return to the surface as springs or rivers. In their wake are the caves, caverns and sinkholes which result from their chemistry.
The affects of the chemical erosion in shaping the landscape of the karst have been more than exacerbated by the physical influences across Imotski. Tectonic forces, including earthquakes, have uplifted and faulted the karst, resulting in fractures that guide water flow and weaken cave ceilings, leading to collapses.
Approaching Imotski
When nearing Imotski the landscape looks greener. This is due to the young, weakly developed soils that are typically found in alluvial or lake deposits. These soils consists of fresh sediments deposited by the water which can transport organic material to the area. The sediments are quite deep in the Imotski plain supporting vineyards and a diverse range of crops. We will consider these in the next post.
The Imotski Sinkholes
Sinkholes are quite common in karst landscapes, although not as large as the sinkholes at Imotski. In fact, one of the sinkholes has been indexed as the world’s third largest sinkhole. The typical model for a sinkhole is that limestone formations are progressively both chemically and physically eroded over millions of years, perhaps forming subterranean caverns. The tops of the caverns and fissures eventually collapse under their own weight and the sinkhole results. Occasionally the process may be aided and abetted by earthquakes or other physical forces. This probably would have been the case with the Imotski sinkholes as the region is tectonically unstable within an active fault zone and has been the subject of earthquakes.
The Blue and Red Lakes of Imotski
The water levels of these sinkholes can vary significantly, with water refreshing the lakes in the winter and being exhausted in dry summers. The levels have been known to vary by over 200 metres.
The Blue Lake
The lake on the immediate margin of Imotski is known as the Blue Lake, or Modro jezero in Croatian.
The lake occupies a chasm with an approximate length and width of 800 metres and 400 metres respectively. The average depth of the Blue Lake sinkhole is stated on the Information board as 220 metres from rim to floor. It’s maximum water depth is said to be 110 metres. Fitter and more adventurous travellers can take some trails down to the bottom, test the temperature of the water in the lake and then have a swim. The lake may dry out in summer making the floor of the chasm available for hiking.
Why is the Colour of the Water Blue?
The ‘blueness’ is due to the scattering of blues in the water. The scattering can be due to the water itself, and due to the presence of salts or particulates.
The scattering of light in water shouldn’t be confused with the scattering of light in the atmosphere. Scattering in the atmosphere occurs when gas molecules of appropriate dimensions absorb the incident light and then re-radiate it. Scattering in water occurs due to the random motion of water molecules. The incident light is subject to changes in water density which dynamically alters the refractive index.
So why are the blues scattered more than the reds? To get an understanding of the reason we have to look at the water molecule itself, and particularly the bonds between the atoms. The bonds between the hydrogen and oxygen atoms in the water molecule vibrate. The vibrations are excited by photon energies that correspond to the energy differences between their quantised vibration energy states. In other words, not any old energy will do. These vibration energies for the bonds of a water molecule correspond to the energies transmitted by the long wavelengths in the visible spectrum, the reds. The bonds absorb the long wavelengths thereby reducing the impact of the reds on the water colour. Think of it as climbing a ladder. If the length of your stride matches exactly the differences in height between two runners on the ladder you can progress up the ladder. The bond vibrations are not affected by the blue light and so the blue light can travel deeper and become a source for scattering. A similar discussion could apply to the intermolecular hydrogen bonds.
The Red Lake
The Red Lake is located about one kilometre out of town.
We have chosen to display an imported image due to the shadows on our own images which reduced the visual effect of the reddish iron oxide.
Peter1936F, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0, via Wikimedia Commons
The Red Lake, or Crveno jezero In Croatian, is also a karst feature and its origin would probably fit the same model as the Blue lake. The Red Lake is located in a depression with a diameter of about 300 metres. The maximum sinkhole depth is about 550 metres with a visible water depth of about 250 metres. The Red Lake is the world’s third largest sinkhole. Although this lake drains via subterranean waterways through the limestone bedrock, it remains hydrated all year. This suggests an inflow into the lake.
So Why do the Cliffs in This Lake Appear Red?
The name of the lake is not determined by the colour of the water but by the colour of the cliffs that surround it. The reddish/brown colour in the walls of the lake is iron oxide, or hematite to name its mineral. The atomic arrangement of iron oxide causes it to absorb the other wavelengths without scattering the reds. The walls appear red because the red light is available to be reflected back to the observer. The reddish-brown trails find their end as a fine sludge at the bottom of the lake. Iron derived particulates suspend in the water ensuring light is reflected upward. The colour of the water might be impacted but certainly not to the extent that would be caused by scattering.
Other Imotski Attractions
Apart from the karst landscape, Imotski offers medieval fortifications in the form of the Topana Fortress that sits on the rim of the Blue Lake. There are also exceptional vantage points for photography, walking and cycling trails, sights of geological interest to explore for those inclined, wineries and a traditional town with stone streets and community squares. Other natural attractions around Imotski will be explored in the following post. One site of particular interest for a day trip from Imotski are the Vrijika River springs where apart from enjoying the pristine scenery and walking trails, take advantage of canoeing adventures.
Conclusion to The Drive to the Imotski Sinkholes from Omis
There aren’t any obvious reasons why travellers can’t enjoy Imotski. We recommend a visit to Imotski because both the drive to Imotski through the karst and Imotski’s attractions engage thinking. They offer something different.
As far as practical matters are concerned, the venue appears to be safe although parents would need to keep children in view near the edges of the sinkholes. The area around the sinkholes has been well developed for tourism and prepared walkways make it easy to skirt around and see the sink holes. Parking should not be an issue in Imotski due to its relative remoteness. And if you are travelling on a budget, Imotski is not expensive
A day visit is suitable and Imotski is littered with natural and geological interest.
With Dubrovnik in mind it is now time to plan a route. But which one? The next post will offer some options.
