As glaciers receded and windblown silt covered their wake, a landscape of wooded river bluffs and rolling loess hills remained, including some of Kansas's steepest hills. Near White Cloud, loess can be more than 100 feet thick, thinning to the southwest, and along the Missouri some bluffs rise 200 feet above the river, including Lookout Mountain southeast of Iowa Point. Four-State Lookout is the most notable Kansas bluff overlooking the Missouri, recognized for decades for its views of Kansas, Missouri, Nebraska, and Iowa. The glaciated region also features large river systems, several natural and man-made lakes, and waterways that mostly drain south to the Kansas River and then the Missouri, though a few streams still drain north toward the Nemaha River in Nebraska. Before the Ice Age, northeast Kansas drained east toward St. Joseph, Missouri through the Grand Valley, but the first ice sheet blocked that outlet, formed glacial Lake Atchison, and, when the lake overflowed, created new drainage ways that became major river valleys including the Big Blue River and the Missouri downstream of Leavenworth. Sediments and glacial deposits later filled the original valley and trapped its water underground, leaving areas along it with especially high groundwater yields that became important to agriculture. A second ice sheet advanced farther south, blocked major river valleys, formed more glacial lakes, and, as they filled and overflowed, eroded new spillways parallel to the Kansas and Missouri valleys, including Mill Creek and the Wakarusa River. Large boulder beds near these valleys show the volume of water these streams once carried, and the region's water access and fertile soil attracted settlers who built communities such as Atchison, Leavenworth, Troy, and White Cloud.