Geology of Fort Larned National Historic Site, Kansas

Cavalry reenactors at Fort Larned. NPS photo.

Fort Larned National Historic Site preserves a military post active from 1859 to 1878, near the midpoint of the Santa Fe Trail in Pawnee County, central Kansas. The site covers 718 acres, mostly federal, including a small detached parcel about 3.5 miles (5.6 km) south that protects Santa Fe Trail wagon ruts. This summary comes from the NPS Geodiversity Atlas and Geologic Resources Inventory.

A fort in a river bend

The fort stands on the floodplain of the Pawnee River, beside a large oxbow — a broad, flat plain of Quaternary sand, gravel, silt and clay. The river and oxbow protected three sides; only the south lay open, and its flat grasslands let sentries see anyone coming.

• The river: it meanders west to east through the site, flowing 198 miles (319 km) from Finney County and draining 2,700 square miles (7,000 km²), and joins the Arkansas River eight miles (13 km) downstream. Its dendritic (branching) pattern reflects nearly flat-lying rock.
• Then and now: once a clear, sandy year-round river, the Pawnee — like the Arkansas — now flows only after heavy rain, because of irrigation.
• No bedrock shows in the site. The nearest outcrop is Dakota Sandstone at Jenkins Hill, about 3 miles east.
• The ruts unit: on gently rolling uplands of loess — buff, limy windblown silt with a little clay and fine sand, typically 6 to 10 feet (2 to 3 m) thick.

The Pawnee River and the oxbow on Fort Larned's east side. NPS photo.

Regional setting

Fort Larned lies in the Arkansas River Lowlands, one of 11 physiographic regions of Kansas: a flat alluvial plain with shallow groundwater. The Smoky Hills, marked by Cretaceous rocks, lie to the north; the vast High Plains cover the western third of the state. Loess blown from glacial debris over the last 12,000 years covers much of the uplands.

Underground water: the Pawnee valley is about 1,000 feet (300 m) wide, with Pleistocene–Holocene alluvium averaging about 105 feet (32 m) thick: silt above, then sand and gravel that yield plenty of well water. The aquifer is recharged by rain, snowmelt, runoff and streamflow. The site's three wells, about 55 to 85 feet (17 to 26 m) deep, likely tap the sandy lower alluvium. Central Kansas is breezy: loess covers the uplands, and there are sand dunes to the southeast.

The Pawnee River

• Before the fort: a single-thread, meandering, somewhat incised stream; its course has held since at least 1859, as a photo of the early Army bridge shows.
• Today: deeply incised, 15–25 feet (4.5–7.7 m) below the valley floor, and 100–200 feet (30–61 m) wide at the bank tops.
• Flow: annual mean flow ranges from 5–6 cubic feet per second in a dry year to over 100 in a wet one; the record was about 550 cfs in 1951. Yearly peaks run from under 100 to over 10,000 cfs. Water runs in the channel in about three years out of four.
• Fewer floods: the 2-year flood at Rozel, about 10 miles upstream, is 2250 cfs, but since the 1960s such floods seem less frequent, perhaps from changing rainfall or a falling water table due to pumping.
• A changed river: its flow, sediment, channel, streamside plants and habitat have changed greatly since the fort's day. A check dam just downstream has altered the channel's gradient and elevation. See the riparian condition assessment.

Building stone

• Walls: mostly Dakota Sandstone, 2 to 3 feet thick — rough blocks in older buildings, dressed and fitted blocks later. The soft, crumbly stone carries historic inscriptions carved by hand.
• Sills and thresholds: many of Greenhorn Limestone. With wood so scarce, its fencepost layer made fenceposts.
• "Kansas marble": possibly Funston Limestone, used in some thresholds and rear walls.

Fort Larned from the air. NPS photo.

Fossils

Pollen has been found in Holocene sediments; no other fossils are known in the site. But the building stone has some: a leaf impression in a sandstone block in the park collection, and bivalve shells in Greenhorn blocks. All NPS fossils are protected under the Paleontological Resources Preservation Act of 2009.

Hazards

• Flooding: possible along the Pawnee, though rare lately; the whole site lies within the 100-year floodplain.
• Earthquakes: central Kansas has low seismic hazard. The USGS 2014 hazard map gives a 2% chance in 50 years of shaking between 2 and 4 %g (about intensity IV), and fewer than 2 damaging shakings expected in 10,000 years.

Sources

Based on "NPS Geodiversity Atlas—Fort Larned National Historic Site, Kansas," National Park Service; a work of the United States government in the public domain. The source converts the walls' 3 feet to 1.2 m, too much, so metric is left out for the walls. More: Fort Larned, the Geologic Resources Inventory and the Kansas Geological Survey's physiographic regions.