The Laurentide Ice Sheet of the last glacial period terminated in several lobes along its southern margin. The timing of maximum extent may have varied among the terminal lobes owing to internal ice sheet dynamics and spatially variable external controls. Some terminal ice lobes, such as the westernmost James Lobe, remain poorly dated. To determine the timing of maximum ice extent in this key location, we have mapped glacial deposits left by the Pierre Sublobe in South Dakota and applied 10Be surface exposure age dating on boulders on moraine ridges associated with three distinct late Quaternary glacial drifts. The oldest and most extensive “Tazewell” drift produced variable 10Be surface exposure ages spanning 20–7 ka; the large range is likely attributable to moraine degradation and subsequent boulder exhumation. The oldest ages of about 20 ka are probably limiting minimum ages for the Tazewell moraine surfaces. By contrast, exposure ages of the youngest “Mankato” drift of the easternmost Pierre Sublobe tightly cluster at about 16 ka. This age for the Pierre Sublobe is consistent with the nearby Des Moines Lobe, suggesting both acted together.

中文翻译：

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James Lobe, Laurentide Ice Sheet 的 Pierre Sublobe 的表面暴露年代测定

末次冰期的劳伦泰德冰盖在其南缘的几个裂片中终止。由于内部冰盖动力学和空间可变的外部控制，最大范围的时间可能在终叶之间有所不同。一些末端冰叶，如最西端的詹姆斯叶，仍然过时。为了确定这个关键位置最大冰层范围的时间，我们绘制了南达科他州皮埃尔亚叶留下的冰川沉积物图，并应用10是与三个不同的晚第四纪冰川漂移相关的冰碛脊上巨石的表面暴露年龄。最古老和最广泛的“Tazewell”漂移产生变量10是跨越 20-7 ka 的表面暴露年龄；大范围可能归因于冰碛退化和随后的巨石挖掘。大约 20 ka 的最古老年龄可能限制了 Tazewell 冰碛表面的最小年龄。相比之下，最东端的 Pierre Sublobe 最年轻的“Mankato”漂移的暴露年龄紧密聚集在 16 ka 左右。Pierre Sublobe 的这个年龄与附近的 Des Moines Lobe 一致，表明两者是一起行动的。