Fold hinge / axis measurements
8,253 fold hinge / axis measurements from published US geologic maps, recorded as trend and plunge and cited to the map each came from — 1% of the Atlas.
Show only fold hinge / axis on the map
What it is
The hinge is the line of greatest curvature on a folded surface, and the fold axis is the line that, moved parallel to itself, generates the fold. For a cylindrical fold the two are the same direction, recorded as a trend and a plunge.
A measured fold axis is the direct version of what a stereonet otherwise has to derive: fit a great circle to a scatter of bedding poles and its pole is the constructed axis. Having both in one place is useful — where a measured hinge and a constructed axis disagree, the fold is not cylindrical, and that disagreement is itself the finding.
Fold hinges are the rarest structure in most published mapping, because measuring one requires an exposed hinge zone. Counts here are small next to bedding, and their distribution reflects exposure as much as structure.
How the Atlas records it
Every fold hinge / axis row carries an azimuth and an inclination in one convention
(trend and plunge), a position in WGS 84 with the publisher's
own location confidence, the raw type term as the source published it, and the citation of the source map.
Selections export as CSV with a companion sources.csv of citations.
Which sources have it
| Source | Measurements | Share |
|---|---|---|
| USGS NGMDB GeMS | 2,066 | 25% |
| USGS SIM quads (NE) | 1,352 | 16% |
| Washington Geol. Survey | 1,309 | 16% |
| NPS GRI parks | 1,023 | 12% |
| USGS data releases · MRDS | 887 | 11% |
| NGMDB legacy quads | 693 | 8% |
| South Dakota SDGS | 567 | 7% |
| Alaska DGGS | 198 | 2% |
| Vermont Geol. Survey | 57 | <1% |
| Pennsylvania Geol. Survey | 48 | <1% |
| Wyoming WSGS | 47 | <1% |
| Oklahoma Geol. Survey | 4 | <1% |
| Oregon DOGAMI (OGDC-8) | 1 | <1% |
| Wisconsin WGNHS | 1 | <1% |
Where it is
| State | Measurements | Share |
|---|---|---|
| Washington | 1,317 | 16% |
| Maine | 1,225 | 15% |
| New Hampshire | 972 | 12% |
| Colorado | 947 | 11% |
| Vermont | 767 | 9% |
| South Dakota | 570 | 7% |
| North Carolina | 377 | 5% |
| Virginia | 286 | 3% |
| Alaska | 273 | 3% |
| Idaho | 229 | 3% |
| Wyoming | 205 | 2% |
| Connecticut | 198 | 2% |
| California | 162 | 2% |
| New York | 157 | 2% |
| New Jersey | 94 | 1% |
| Minnesota | 75 | <1% |
| Pennsylvania | 74 | <1% |
| Maryland | 54 | <1% |
| Tennessee | 53 | <1% |
| Montana | 48 | <1% |
| Georgia | 46 | <1% |
| Utah | 39 | <1% |
| Arizona | 35 | <1% |
| Massachusetts | 8 | <1% |
| Oregon | 5 | <1% |
| Oklahoma | 4 | <1% |
| Texas | 2 | <1% |
| Wisconsin | 1 | <1% |
| New Mexico | 1 | <1% |