The age and origin of the Grover Gravel, which is preserved on stable uplands in St. Louis County, Missouri, USA, and similar gravels along the Mississippi River bluffs farther south, have been controversial for over a century. These gravels have some features consistent with a glacial origin but lack obvious erratics such as igneous rock fragments. New observations, however, show that the earliest tills within the Mississippi basin nearly lack such constituents as well, and that the Grover Gravel is a Pleistocene deposit derived from such a till. A Mississippi River terrace deposit in St. Louis County also includes typical clasts of this till and formed between 2.0–1.8 Ma, based on various dating methods. This deposit may correspond to a pulse of glacial meltwater recorded within sediment near the mouth of the Mississippi River and represent outwash from a previously unrecognized glaciation within the US midcontinent. The Mounds Gravel (or Upland Complex) is present farther south around the perimeter of the Mississippi Embayment and contains the same distinct clasts as the gravels in St. Louis County. Given that the age of the Mounds is <~3 Ma based on cosmogenic‐nuclide ratios, the southern gravel could be the same age as the gravel terrace in St. Louis County. If so, these gravels could mark a geomorphic transition to glacial‐dominated sedimentation farther south in and around the Gulf of Mexico. Regardless, all the upland gravels have insignificant amounts of zircon from provinces west of Hudson Bay. Therefore, detrital zircon provenance does not support a recent hypothesis that the Mounds (Upland Complex) was deposited by a giant Mississippi River with a drainage basin extending across broad areas west of Hudson Bay.

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美国密苏里州山地砾石的年龄和出处及其与早更新世冰期的关系

在美国密苏里州圣路易斯县稳定的高地上保存的格罗弗砾石的年龄和起源，以及在更南端的密西西比河悬崖上的类似砾石一直存在争议。这些砾石具有与冰川起源相一致的某些特征，但缺乏明显的不稳定因素，例如火成岩碎片。然而，新的观察结果表明，密西西比盆地内最早的耕作也几乎缺乏这种成分，而格罗弗砾石是这种耕作的更新世沉积物。根据各种测年方法，圣路易斯县的密西西比河阶地沉积物也包括该耕作的典型岩屑，形成于2.0–1.8 Ma之间。该沉积物可能对应于密西西比河河口附近沉积物中记录的冰川融水脉动，代表了美国中部大陆先前未被识别的冰川作用引起的冲刷。土墩砾石（或山地综合体）位于密西西比河沿岸外围以南，与圣路易斯县的砾石具有相同的碎屑。根据宇宙成因核素比率，考虑到土墩的年龄<〜3 Ma，南部砾石的年龄可能与圣路易斯县的砾石阶地相同。如果是这样的话，这些砾石可能标志着向墨西哥湾及其周边更南端的冰川控制的沉积过渡。无论如何，所有的山地砾石都含有少量的哈德逊湾以西各省产的锆石。因此，