Recently, I toured a number of geological and historical sites between the suburbs of St. Petersburg and the Valdai Hills region. As one who lived and learned geology in semi-arid, mountainous regions of the American west, this trip offered a fresh look at surface geological processes, as well as a new appreciation for Quaternary geology. The Quaternary period spans the past 2.588 million years and includes the Pleistocene and Holocene epochs, but most research focuses on climatic and geographic changes during the latest Pleistocene “ice age” and the Holocene “interglacial” (11,500 years ago until present). Northwestern Russia contains many pristine records of both intervals and so is a frequented location for those wanting to reconstruct the past ~100,000 years of Earth history.
|Figure 2: Outcrop of Early Ordovician sandstone and shale (top layer) on the Izhora Plateau. This sandstone contains numerous cross-bed sets that resemble modern beach sediments. Traditionally, this outcrop has been interpreted as a transition from the littoral zone (i.e. ‘near the beach’) to a lagoonal environment.|
Of course, the geological history of this region begins much earlier. Underlain by igneous rocks that comprise the ~3-billion-year-old Baltic Shield, layers of sedimentary rock were deposited between the Late Neoproterozoic (Ediacaran Period) and the Middle Paleozoic (Devonian Period). By and large, these sedimentary rocks—sandstone, shale, and limestone—contain marine or coastal fossils and are typical of a passive margin. For reference, think of the modern southeastern coast of the United States, which gently slopes into the Atlantic and Caribbean seas and is now accumulating fine-grained sandstone and limestone with marine fossils.
|Figure 3: Outcrop of Devonian carbonate rocks on the shore of Lake Ilmen. Several layers are very rich in marine molluscs (see Fig. 10), providing evidence to their depositional environment.|
If you live in the western U.S. or Canada, you may already be familiar with this geological sequence. There, Late Neoproterozoic to Middle Paleozoic sedimentary rocks overly very old granite and granodiorite (2.7–1.8 billion years old). These rocks comprise the “basement” of western states from Idaho/Nevada to Montana/Wyoming. Hence the geological record in Idaho, for example, is quite similar to that of northwestern Russia.
The rock records of both the western U.S. and northwestern Russia are explained geologically by modern analogs of coastal environments, along with the theory of Plate Tectonics. The spreading of ancient seas (not unlike the modern Atlantic) is a major long-term control on subsidence and relative sea level. Subsidence refers to the ‘sinking’ of crustal rocks with respect to sea level as 1) the basement rock cools with time to become more dense, and 2) accumulating sediments add weight to the crust, which is essentially ‘floating’ on the mantle. Slow rates of subsidence (on the order of millimeters per decade) allow for the accumulation of marine sediments on the edges of large continents over long periods of time.
|Figure 5: Nearly hidden outcrop of Ordovician limestone in a small depression formed by recent tectonic activity.|
|Figure 6: Closeup of features in the outcrop above. A) Clear view of brittle deformation in the limestone. According to the Young-Earth timeline, these rocks must have been deformed only a few hundred years after being deposited (during the “Post-Flood Ice Age”). Yet by the time of deformation, the lime sediments must have been completely cemented and lithified to form such brittle fractures. Unfortunately for the YEC, deeply buried, water-saturated sediment does not behave like fresh-laid concrete in the driveway. The proposed timeline is rather preposterous. B) Abundant trace fossils of organisms living in shallow marine waters. C) These burrows reach up to ~1 foot in length and run horizontally through the rock. Horizontal burrows are typical of arthropods that feed in the shallow sediment. In other words, these are not tracks of organisms desperately trying to escape during a catastrophic flood. Else where are the all the critters themselves?|
Since subsidence rate is the ultimate control on sedimentation rate, one can begin to understand why the sedimentary record seems so ‘patchy’ and why sedimentary contacts (including unconformities) are often so ‘flat’. If high-energy weather events or even local catastrophes (e.g. tsunamis) deposit large quantities of sediment in the shallow sea, more common forces like waves, tides, and gravity work diligently to flatten out the seabed. Part of the event (fossils included) will get preserved in the rock record, while the majority is ‘washed out to sea’. In other words, it is entirely possible to bury organisms and sediment forms rapidly in a ‘uniformitarian’ setting (Fig. 6).
|Figure 7: Purely for scenery; view of the small lake adjacent to the outcrop above. Although crystal clear, the lake is devoid of animal life because… (below)|
Quaternary sediments lie directly on top of Paleozoic rock (Devonian or older) in this region. Whatever geological events transpired in northwestern Russia between the Devonian and the Pleistocene may ever be a mystery to us. Not because the geologic column is a sham, as YEC’s like John Woodmorrape spuriously claim, but rather because sediments from those intervals have been wiped away from the continent (we know this partly because their remains are found among Quaternary ‘debris’, but also because rocks from that period are present in other parts of Russia). The mechanism responsible was driven by climatic events that dominated the latter half of the Pleistocene. During this time, the Russian landmass was already situated in high latitudes of the northern hemisphere and provided a foundation for advancing sheets of ice.
As the global climate cooled repeatedly by more than 10°C, these ice sheets grew up to several thousand meters thick and literally ‘bulldozed’ whatever laid in their path. Some rocks were even ground into fine powder and deposited in lakes and river beds that formed in front of the wall of ice. Under the weight of the massive ice sheet, the entire north-Eurasian landmass was compressed and downwarped. The downwarping caused many Paleozoic rocks to be folded and fractured. It was so extreme that parts of Scandinavia have rebounded in elevation more than 500 meters since the disappearance of the ice (to which modern fjords provide stellar visual examples), and are still ‘recovering’ today.
|Figure 12: Maximum extent of the most recent ice sheet over northern Eurasia.|
In North America, the last ice age is called the Wisconsin glaciation—named after the locality that marks its maximum extent. The same period is called the Valdai glaciation in Russian terminology. Thus my tour ended up in the Valdai Hills, which are home to the terminal moraines of the last glacial maximum.
|Figure 14: View of Lake Valdai, a remnant of the glacial landscape, from the bridge connecting an island monastery (below) to the mainland.|
About 14,000 people live in the city of Valdai—a popular vacation spot and home to many summer cottages (including one that belongs to Mr. Putin). Even the Fall scenery of this cozy settlement has much to offer, in my opinion, despite the constant rain and slightly ‘chilly’ weather.
|Figure 15: View of Lake Valdai from the northern shore; Iverskiy Monastery visible on opposite shore.|
|Figure 16: Main cathedral of Iverskiy Monastery.|
In the mid-17th century, construction of Iverskiy Monastery took place by order of the Patriarch Nikon. The main cathedral (Fig. 16) was built in only two summers and closely resembles the architectural style seen elsewhere in Russia during this period. With the exception of the Soviet era, the monastery has functioned since the 1650’s. As I recall from the tour, this plot of land survived in part because it was used as a recreational camp for Soviet youth. Regardless, renovation of the various cathedrals, clerical living quarters, dining hall, hospital, and towers has been ongoing since 1991, when the property was returned to the Orthodox church.
|Figure 16: Who doesn’t love a good mushroom hunt? They say all mushrooms are tasty, but some only once.|
|Figure 17: I didn’t take my chances with this mushroom either…|
After touring the monastery, we managed to take a relaxing walk through the hills adjacent to the lake. For me, the stark contrast in vegetation to my childhood in Coloradan forests was most exciting. Here, the forest floor is soft and thick, full of ferns and other plants that simply don’t grow in such arid conditions. It may sound strange—well, it is—but I also did not understand the concept of a “mushroom hunt” until now. These things grow everywhere, and quite large!
|Figure 19: Tread lightly. The floor of this marsh lies several feet beneath what only looks like grass.|
As the Scandinavian Ice Sheet advanced toward Valdai, the constant melting of ice at its margin produced torrents of sediment-choked streams. Modern sandurs, or glacial outwash plains, are best known from Iceland. They are dynamic landscapes that are reshaped constantly by deposition of sediment that was eroded from the continent and locked up in the ice. Sandurs are preserved in the geological record as thick beds of cross-bedded sand and gravel (Figs. 20–22), due to braided rivers that sweep across the plain in front of the glacier. In fact, the weight of the glacier often forms a ridge many miles in front of the ice (imagine stepping on a floating log to raise the opposite end) that keeps these rivers flowing parallel to the ice margin rather than away from it.
|Figure 21: More of that horizontal bedding, though with some coarser grained beds. One can imagine the high energy of flow and massive amounts of water being dumped into the basins at this time.|
|Figure 22: Closer view of channel cross bedding; very typical of modern braided streams.|
As the Scandinavian Ice Sheet began to melt and recede from the landscape, it left clear evidence of its path. Numerous lakes formed in depressions left by the ice. Erratic boulders carried hundreds of kilometers by the ice now dot the surface. Well, it’s not unlike any other glacial landscape, I suppose. But it was my first experience with such landforms and deposits in person!
|Figure 27: When clay accumulates ‘quickly’, it retains excess water that somehow must escape. These minor folds in the varves (called load structures) are evidence of that escape.|
Thus far we covered the bedrock stratigraphy from St. Petersburg to Valdai, which recorded deposition between the Ediacaran and Devonian periods in a passive marine margin. After that, we looked at Late Glacial deposits in and around the Valdai Hills region. In the second part, I’ll post pictures from along the glacier’s retreat (Lake Ilmen back to St. Petersburg) and discuss some of the Holocene changes that took place in northwestern Russia.
Feel free to post questions or comments below regarding any of the pictures and discussion.