John W. Weymouth
University of Nebraska
January 31, 1997
Fort Clatsop near the town of Astoria, Oregon is the site of Fort Clatsop erected by Lewis and Clark in 1805. This site has been tested by excavation at various times. There is a reconstruction of the Fort at the Memorial but the precise location of the Fort is not known. As part of the ongoing archaeological study of the Fort, the Fort Clatsop contracted to have a magnetic survey conducted over part of the site area in 1996. The survey covered about 600 square meters south-west and about 200 square meters north-east of the reconstructed fort. This is a report of that survey.
On November 7, 1805 the Lewis and Clark expedition reached the estuary of the Columbia River (Ferris, 1975). For several days, and in bad weather, they explored the north shore of the estuary for a suitable winter camp site. Finding the north shore inhospitable and lacking in game they crossed over to the south shore on November 26 and eventually on December 7 they located a site 3 miles up what is now known as the Lewis and Clark River. They started to build a fort Called Fort Clatsop after a local Indian tribe. It was a 50 feet by 50 feet enclosure with two rows of cabins inside facing each other. Although not finally completed until December 30 the men moved in on Christmas eve.
The Lewis and Clark expedition left Fort Clatsop March 23, 1806 for their return trip. The abandoned fort was all but gone by 1850's. In 1852 a Mr. Shane, who built a cabin at the site, reported that there existed remains of two of the Lewis and Clark cabins. In June and July of 1948 Caywood conducted some exploratory excavations. He reported that there was evidence of "charcoal and then later some thin layers of orange-red burned earth and burned stones of about the size of baseballs and larger". These excavation units are indicated on the GIS maps produced by Keith Garnett, 1996 (referred to here as the KG maps). Schumacher dug several trenches in 1956, 1957 and 1961. These are marked on the KG maps.
Today, in spite of several archaeological studies the exact location of the Fort is unknown. A exhibit of the fort was built on the site in 1958 by a local civic group. In 1958 the National Park Service acquired the site and named it the Fort Clatsop..
James Bell (Bell, 1990) conducted some radar surveys at the fort site in 1990. In 1996 (Bell, 1996) he conducted a more extensive survey over regions south-west and north-east of the fort exhibit. These transects as well as areas of possibly significant reflections are marked on the KG maps.
In general a magnetic survey consists in measuring the magnetic field of the earth a few centimeters above the surface on a grid of evenly spaced points. Slight differences in concentrations of wealdy magnetic iron oxides beneath the surface can give rise to anomalies in the mapped data. Such concentration differences can have anthropogenic causes such as filled pits, fired or burned earth, intrusive walls and cellars or privies (Weymouth, 1986).
There are two ways to measure the earth's field, total field or gradient. In the total field method the magnitude of the field, regardless of direction, is measured. In the gradient method the gradient of the field or the difference between two readings separated by a short vertical distance is measured. The fluxgate gradiometer measures the gradient of the vertical component of the field. Because the small anomalous field caused by a local concentration of magnetic soils decreases rapidly in strength with distance from the source the gradient is strongly influenced by sources near the gradiometer whereas the signal from more distance sources is almost canceled. Thus a gradiometer emphasizes near surface features.
A Geoscan Fluxgate Gradiometer, FM-36 was used in the survey. The lower sensor of the pair of sensors was about 30 cm above the surface while the upper sensor was 50 cm above the lower sensor. Measurements were made at 1/2 m intervals on traverses separated by 1/2 m. The surveyed region was separated into 10 m by 10 m blocks with 400 data points (or sometimes less) in each block. Each full block took 25 to 30 minutes to survey. Soon after data were obtained preliminary maps were generated using the Geoscan software Geoplot. Subsequently more complete maps were plotted using the programs Golden Surfer or Fortner Transform. Outlines of the anomalous areas extracted from the magnetic maps were passed to Keith Garnett and placed in the GIS map files for subsequent plotting.