= Solution
For the two negative <gravity anomalies> in the actual Figure 1, examine \b[where the steepest flanks occur within the total fall into the trough]. The solid upper intrusion profile has maximum slope relatively low on its flanks, close to the flat trough; its body widens downwards. The lower basin profile has maximum slope relatively high on its flanks, near the shoulders; its body narrows downwards. The dashed gradient traces and arrows mark these transitions. The signed <derivative> changes sign on opposite sides of a symmetric trough; a plot of gradient magnitude instead gives two positive peaks.
A shallow edge controls a relatively abrupt local change, whereas deeper <geophysical density contrast> spreads its effect over greater horizontal distance. Downward-narrowing geometry leaves relatively more of the anomaly associated with the shallow broad part, placing the strong flank change nearer the outer shoulders. Downward-widening geometry adds a broader deep contribution beneath the narrow outcrop, shifting the inflection towards the lower part of the trough. Thus the location of a flank's maximum slope relative to its shoulder and minimum is more useful here than the shared $-40\ \mathrm{mGal}$ peak alone.
A symmetric constant-density body with \b[vertical sides] gives a symmetric negative trough with paired rounded flanks; for a broad shallow body the main slope changes occur approximately halfway down those flanks. It is intermediate between the two end members, rather than a triangular anomaly or a rectangular discontinuity. Exact fractional inflection heights depend on width, depth, burial and rounding. The following independently calculated infinite-strike models illustrate the distinction without copying Figure 1.
\Image[/past-exam-of-the-year-2-of-the-earth-sciences-geology-course-of-the-university-of-oxford/2024/paper-3-q2-gravity-shapes.png]
{title=Gravity-trough shapes for bodies narrowing downward, having vertical sides, and widening downward}
{description=The bodies have the same vertical thickness and negative density contrast; each profile is normalized by its own central anomaly. Markers identify maximum flank slopes. A small burial depth regularizes the near-surface edge response.}
This qualitative inference assumes approximately comparable simple bodies, uniform <geophysical density contrast>, and a properly removed regional field. <Gravity inversion> is non-unique: lateral <density> changes, multiple bodies, and a buried top can imitate changes of wall slope. Use the <seismic reflection> constraints to test the interpretation.
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