On 22 April 1805, Captain Meriwether Lewis was hiking the bluffs of the Missouri river near present-day Williston, North Dakota. Sixty metres below him, other members of the Lewis and Clark expedition were waitingfor strong headwinds to die down so they could continue their laborious journey up river. Lewis decided to explore on foot. As he walked, he spied an outcrop of polished rock, strewn with pebbles that were smooth and worn, as though tumbled in a mountain stream. Lewis concluded that the water had once been all the way up there. We now know that Lewis’s ancient beach was created a million years ago, before ice-age glaciers blocked an earlier river and forced the Missouri to carve its present, much deeper, valley. But geologists are astounded that Lewis, with no knowledge of ice-age phenomena, realised how the ledge was formed.lt is one of many spot-on deductions that reveal that he and William Clark were remarkably good intuitive geologists.
IN THE early 19th century, geology was barely a science. There were no grand theories, no concept of the processes that build mountains, barely an inkling of the geological timescale and the meaning of fossils. But in 1803, when President Thomas Jefferson ordered an expedition to explore the vast Louisiana territory that the US had just bought from France, he could not have picked better geologists to lead it than Meriwether Lewis and William Clark. From March 1804 to September 1806, the two men kept their eyes peeled, took meticulous notes, and drew conclusions that often required a great deal of insight. It is only now, two centuries after they set out, that geologists are beginning to recognise their accomplishments.
For generations, the explorers were maligned by historians, who gave them credit for wonderful descriptions of botany and geography but dismissed their geological observations as worthless. “These historians didn’t realise they were reading geology in the journals of Lewis and Clark,” says John Jengo, a consulting geologist from Downingtown, Pennsylvania, who has been fascinated by the expedition for 25 years. “In very simple phrases, Lewis and Clark communicated concepts that are obvious to a geologist.”
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One of the expedition’s more interesting discoveries was finding the source of the “pumice” that earlier explorers had seen floating far down the Missouri river. Reports of the floating stones may even have reached Jefferson, who – perhaps worried that he’d bought nothing but a vast, lava wasteland – had specifically instructed the explorers to keep an eye out for volcanoes.
In the spring of 1805, Lewis and Clark traced the stones to outcrops in the river bluffs of western North Dakota. Then, in one of those intuitive flashes that typified the explorers’ geological acumen, Lewis realised that the “pumice” was found only in gaps in coal seams – and that this meant their origins had nothing to do with volcanism.
At the time, nobody knew why rocks formed in layers, but Lewis and Clark clearly understood that strata could be traced from one outcrop to another. Lewis observed that the pumice occurred only where there should have been coal, and that where there was pumice, there was no coal. He correctly concluded that the pumice-like rocks were formed when coal seams caught fire and cooked the overlying rock into something that we would call clinker.
He showed similar insight when he penned one of the earliest known descriptions of petrified wood. “I met with several stones today that had the appearance of wood first carbonated and then petrefyed by the water of the river,” he wrote. Not only is this description perfect – the petrified wood of the western Dakotas is made of coal beneath a surface that looks so much like wood that it is possible to mistake it for weathered timber – but Lewis was also close to the mark in describing the process that formed it. The main thing he missed was that the wood was petrified not by river water but by the slow action of percolating groundwater.
Elsewhere in their journals, Lewis and Clark correctly inferred that muddy tributaries of the Missouri had their sources on the Great Plains, while clear water sprang from the mountains. When they reached the mountains, they reported “shattered, confused, and broken masses of stone”, and hillsides “that appeared to be broken by some convulsion” – not a bad description of the faulting processes now understood to have formed them.
Another major observation came on the Columbia river, slightly upstream of the present-day Bonneville dam. Here, the explorers found a calm pool of water in which the trunks of dead firs rose out of water 6 metres deep. Aware that the trees couldn’t have grown in the water, Clark figured that something must have dammed the river and flooded the forest that once cloaked its banks. He also noted that the current was curiously sluggish – a sure sign to experienced river runners of large rapids lurking downstream. When their boats finally reached the swathe of boulders that had blocked the river and created the rapids, Clark speculated that it had been formed by a gigantic landslide.
Generations of subsequent geologists were to stare at the same rapids and offer conflicting explanations. Some said the dam was produced by tectonic warping or slippage along a fault line. Others thought it was a glacial moraine, debris dumped by a vanished glacier. The debate raged until 1916, when it was confirmed that Clark’s off-the-cuff answer had been the right one.
Present-day researchers have also begun to realise that the explorers’ journals provide a unique, enormously detailed snapshot of the land before America’s westward expansion. Specialists in many fields are now using that information to determine exactly how the land has changed. Some of their findings are surprising.
Conventional wisdom has it, for example, that 20th-century flood-control projects on the Missouri river increased its current by funnelling water more swiftly downstream. Not so, says Dale Blevins, of the US Geological Survey in Lee’s Summit, Missouri. Clark measured the river’s current by anchoring a boat in mid-channel, tossing overboard a log tethered to a rope, and timing how long it took for this “log line” to pay out. Presuming that the rope was the standard 50-fathom (91-metre) line generally used for this purpose, Blevins calculates that the currents Clark measured flowed at speeds of between 9 and 16 kilometres per hour. That is slightly faster, not slower, than those seen at the same locations today. This finding is important to conservationists attempting to save endangered species of fish because it reveals the natural conditions to which these species were adapted.
Clark also measured the river’s sediment load. An old joke holds that the untamed Missouri river was “too thin to plough, too thick to drink”. In fact the expedition did force down its muddy waters. “The water we Drink… contains half a Comn Wine Glass of ooze or mud to every pint,” complained Clark. Leaving aside the question of why the expedition carried a wine glass, Blevins (after researching the size of wine glasses in 1804) concludes that the water must have been 4.5 per cent sediment. Modern sediment levels never exceed 1.5 per cent. Where did all that mud come from? Clark reported that the fast-moving river was eroding sharply into its banks at each turn.
Alex Philp, who develops software for geographical information systems, believes that modern researchers have barely begun to dig information out of Lewis and Clark’s journals. Their expedition took place as the Little Ice Age was ending, and the explorers’ meteorological observations and detailed river descriptions provide an untapped source of data for observing the effects of a change of climate on two of North America’s greatest river basins: the Missouri and the Columbia. Even the plant specimens they brought back provide useful clues. These samples still exist, and researchers have begun to analyse isotopes in their tissues to determine growing conditions and atmospheric conditions in the regions where they were collected.
This type of data may be Lewis and Clark’s greatest contribution to science. “Our understanding of the Earth changes, but as long as someone made very clear observations, you can always get useful information from them,” says Edward Murphy, author of a guidebook to a portion of their route. “What surprises me is what good observers Lewis and Clark were. That has always been one of the best traits a geologist can have.”