Cottonwood Encounters

Cottonwood leaf in mud.

Cottonwoods are common in the American West, even in the drier parts (they often mean water's close by).  I was pleased to meet three on my recent trip in eastern Utah.  The first was an old friend.  The second I've seen occasionally and always loved for it’s beauty.  The third was a complete surprise – new to me even though it’s quite old.

Cottonwoods lined the narrow road down Sheep Creek Canyon on the north side of the Uinta Mountains.  Some were golden, some were bare.  I assumed they were the narrowleaf cottonwood (Populus angustifolia), which is common in the Rocky Mountains, but when I looked close I found my old friend the lanceleaf cottonwood – same as the tree I’ve been following this year (Populus x acuminata).

Still fall in Sheep Creek Canyon ... but it's going fast.

Lanceleaf cottonwood grows at scattered locations in the Rockies.  It was described as a new species by Per Axel Rydberg in 1893, but has since been demoted to hybrid status – a cross between narrowleaf cottonwood and Plains cottonwood (P. deltoides).  Supposedly there’s no evidence that the lanceleaf reproduces by seed.  However it's been found hundreds of miles from its parents, suggesting that it’s self-fertile and “growing from its own seed” (more here).  Similarly, I saw neither narrowleaf nor Plains cottonwoods in Sheep Creek Canyon.  Perhaps Rydberg was right.

Lanceleaf cottonwoods with 300-million-year-old sandstone in Sheep Creek Canyon Geological Area.

I met the second cottonwood species on the south side of the Uinta Range, at the old homestead of Josie Bassett.  Josie staked a claim on the land in 1913 ... when she was 40, divorced, and with kids all gone.  She built a house, developed the cattle operation, and ran it alone until shortly before she died at 90.  The setting is spectacular.  The house sits at the mouth of a box canyon in steeply-tilted and intricately-carved Navajo sandstone.  Nearby I found the most beautiful cottonwood I’ve ever seen, in full autumn glory.  It was a Fremont cottonwood, glowing in the morning light.

Deciduous trees at the mouth of the box canyon mark the Bassett ranch.  Note golden cottonwood on right.

Populus fremontii, part of explorer John C Fremont's botanical legacy.

Fremont cottonwood has distinctive broad leaves with coarse rounded teeth.

My third cottonwood encounter was totally unexpected.  Actually I didn’t see the trees, but their leaves were unmistakable.  They lay perfectly preserved in 47-million-year-old mudstone of the Green River Formation (photo at top of post).

The great lakes of mid-Eocene time.  Lake Uinta was 270 across at maximum extent.

Apparently cottonwoods also were common 47 million years ago, back when the Uinta Mountains were bordered by large shallow lakes.  The climate was semi-tropical, perhaps like Florida's.  For 15 million years, fine sediments and debris containing plant and animal remains were deposited on the lake bottoms.  Now the sediments are rock – the Green River Formation – and the organisms are remarkably detailed fossils.

State Fossil of Wyoming – Knightia.  Source.

I was well aware of the abundant fish fossils of the Green River Formation.  They’re so common that one is the state fossil of Wyoming.  But I didn’t know about the plant fossils until I entered the Eocene Gallery at the Utah Field House of Natural History State Park Museum in Vernal.

A wall covered in plant fossils!

The leaf display was astonishing.  These leaves ended up on a lake bottom 47 million years ago, but they could just as well be last year’s, based on appearance.

This leaf fell off a tree 47 million years ago!

Fossilized sycamore leaf.

The scouring rush (Equisetum) below looks like our modern day ones.  Note the tiny teeth on the dark bands – the free tips of fused leaves – and possibly the remains of a cone at the top.  Below it is a modern-day scouring rush, also with a dark band of fused leaves, and a terminal strobilus (cone).

Photo by Bruce Leander via the Lady Bird Johnson Wildflower Center.

Slab of Green River mudstone with leaf fossils.

I was impressed by the the Field House.  How many communities the size of Vernal (9100 people) have a museum of this quality?  And why Vernal?  Because it sits in the heart of the Dinosaur Triangle, with a fossil record that is "embarrassingly rich" (not just dinosaurs). But it's mission is broader.  It includes interpreting the 80-mile radius around the town, including the Uinta Mountains.  I took in all the exhibits, covering geologic history from Precambrian to present.  They're modern, interesting, clear and justifiably enthusiastic ... if one takes time to read and look carefully.  A booklet explaining the exhibits in more detail is available at the front desk (worth the extra $2.50).

Stylinodon hangs out by a cottonwood tree on the shores of Lake Uinta.

Sources

Bennis-Bottomley, MB.  2012.  Fossil tales; an in-depth guide to the Utah Field House of Natural History exhibits.  Utah State Parks and Recreation.

Sprinkel, DA, Park, B and Stevens, M.  2000.  Geologic road guide to Sheep Creek Canyon Geological Area, northeastern Utah; in Anderson, PB and Sprinkel, DA, eds., Geologic road, trail and lake guides to Utah's parks and monuments.  UGA Publ 29.