I felt it would be wrong at a NJ Audubon Nature Center to chop a plant off in the middle right next to the trail :-(

all I've seen so far are the types of moths that live in leaves, and burrow partially out and make the pupae sticking half out of the leaf.

of the elliptical gall moth is explained here (another example of a fatter gall than yours or the other BugGuide image). It pupates in the gall, and I believe the other gall-forming moths do as well.

I don't know for sure that yours isn't that species. I'm just pointing out that there are other options. Yours actually looks a bit more elliptical than the one already in the guide.

added

The one pictured above has multiple prominent parallel veins, as opposed to just a midrib. The attached photo does look like Euthamia (sources calling it Solidago graminifolia are out of date).

The first plant image may be from my Asteromyia carbonifera shots, I posted another image of the gall/plant.

I'm inclined to guess Solidago canadensis, but if John Pearson disagrees I'd go with what he says.

sorry about the confusion, I should have paid more attention and not assumed there was only one goldenrod along that section of the trail.

I just now discovered this discussion as I was reviewing older pages this evening (I missed lots of BG postings in September due to frequent traveling). In reading over all the related pages, I gather that everyone has figured out that grass-leaved goldenrod, lance-leaved goldenrod, Solidago graminifolia, and Euthamia graminifolia all refer to the same plant species; Euthamia graminifolia is indeed the modern scientific name, but there is much historical usage of Solidago graminifolia.

Unfortunately the additional image of the plant posted by John Maxwell (presumably in the Frass section) appears to have timed out and vanished in the month it took me to find this page, so I can't see it to make an assessment. If you want to re-post another image, I would be happy to take a look. It would be useful to see a zoomed-out view of the whole plant, including flowers if possible.

the sequence went something like I shot this gall on goldenrod, I shot a different gall on goldenrod, then thought to take reference shots and shot a few of the second plant. When I loaded them we realized there were more than one species of goldenrod along the trail that day, and this was the only shot I had of this type. I may have another slightly less cropped, but not much if anything.

Plantwise, this zoomed-in view shows a small segment of stem and the bases of the leaves, to which plant keys pay little or no attention. Nonetheless, I spent some time looking up images and descriptions with an eye for these obscure features. Like Charley, my first impression was that these leaves were too wide to be those of grass-leaved goldenrod (Euthamia graminifolia) because the ones I typically see here in Iowa are narrow. However many of the images available from reputable sources online show leaves that are wider than my personal experience with the plant. Also, upper leaves are narrower and more numerous, easily masking the lower, wider ones - near where the gall is likely located - from casual observation. This description from missouriplants.com contained the most useful description of stems and leaf bases (I've selected only the relevant features):

Stems...short pubescent, minutely winged (ribbed) from leaf bases...

Leaves...3-5 nerved, entire, slightly scabrous...8-9mm broad, glandular-punctate (use a lens to see).

I can see all but the glandular-punctate feature in John's photo. In light of that plus the allowable leaf width, I would say that your photo could indeed be of grass-leaved goldenrod.

Added the most uncropped version that I have that is definitely the plant with this gall.

Building on what I observed in my note above, I can now see more of each leaf, including enough of the blade to test the "veins parallel" trait of grass-leaved goldenrod versus the "veins diverging" trait of Canada goldenrod. These show parallel venation consistent with grass-leaved goldenrod.

Interesting, I don't think I've never seen Euthamia with any prominent veins other than the midrib. Well, Felt (1) divides goldenrod stem and root galls into two sections: "Solidago graminifolia" and "Solidago, Various species." There is nothing that comes close in the former. In the latter, here are the options that could possibly describe this gall--an estimate of size would be useful:

Oval stem gall, length 1/2 inch, caterpillar, summer: Scarred goldenrod gall, Eucosma scudderiana Clem. [now Epiblema scudderiana. The HOSTS database lists the following as hosts for this species: Euthamia graminifolia, Heterotheca subaxillaris, Solidago sp., Solidago canadensis, Solidago gigantea, Solidago juncea, Solidago nemoralis, Solidago ulmifolia. Miller (1963) has a photo of Epiblema desertana galls on Solidago graminifolia, its only host; they are much narrower than those of E. scudderiana on S. altissima.]

Oval stem gall, on S. sempervirens, caterpillar, summer: Gnorimoschema salinaris Busck

Fusiform stem gall, length 1 1/2 inch, usually near ground, on S. latifolia, S. caesia and Aster divaricatus, caterpillae, summer: Gnorimoschema gallaeasteriella Kell.

Elongate, spindle-shaped stem gall, on S. canadensis, caterpillar, summer: Elliptical goldenrod gall, Gnorimoschema gallaesolidaginis Riley [also in S. altissima according to Miller, 1963].

Miller (1963) states that "Galls of G. gallaesolidaginis (fig. 3a, 4) and related species can be easily distinguished because they possess exit bungs which the other galls lack and because they are characteristically shaped. . . The other Gnorimoschema species producing stem galls on goldenrod in Ohio affect different species of goldenrod than the two which G. gallaesolidaginis affects. Indeed, finding a Gnorimoschema gall on a goldenrod other than S. altissima or S. canadensis is reliable evidence that it is not a G. gallaesolidaginis gall. Also, the whitish exit bung of the G. gallaesolidaginis gall (fig. 3a, 4) can serve in the field to separate this gall from those of its Ohio relatives, which make dark brown exit bungs." This "bung" business is explained in Miller's life history section: "Pupation takes place inside galls in July. A month or so later, adults emerge and evacuate their galls through a previously prepared exit. . . In June, the larva begins preparing the moth exit. At some point in the upper third of the gall chamber, a passageway just large enough for the adult is eaten through the wall nearly to the outside. A thin, translucent cap of outer plant tissue is left uneaten. The larva then makes a hard, honey-colored bung which seals this passageway. By the time the bung is completed, the cap of plant tissue has deteriorated. The newly exposed bung soon weathers to become whitish in color (fig. 4)."

Based on all this, I'm thinking Epiblema scudderiana for this one, and the image Max linked to below seems to be G. solidaginis, although the shape still doesn't look right to me. Any other thoughts?

Depending on the source the name changes http://www.google.com/search?sourceid=navclient-menuext&ie=UTF-8&q=Lance-leaved+Goldenrod
I thought it was similar to this, but can't tell the difference

I'm not sure that one's identified correctly. I think a lot of people assume that if it's not the "ball gall" made by the tephritid, it must be Gnorimoschema gallaesolidaginis, not realizing there are several other moths that cause swellings in goldenrod stems. This one looks identical to the one illustrated in Felt (1); I think this page is probably trustworthy.