尽管砂岩墙看起来很坚固, they can be riddled with their residents’ long-lasting residue

科罗拉多高原, 在科罗拉多西部, 犹他州东南部, 亚利桑那州北部和新墨西哥州西北部, is commonly called red rock country or canyon country, both referring to formations of colorful sandstone. 它是西部一个浪漫而风景优美的地方, 有高高的台面, 陡峭的悬崖, 深谷, 狭槽峡谷, 石缝, 拱门和桥梁.

Sandstone that forms on the bottom of a sea has precisely defined, 平行层, while cross-bedded sandstones are fossilized sand dunes. Chinle, 温盖特, Kayenta和Entrada砂岩, formed in the Triassic and Jurassic Epochs between 225 and 140 million years ago, are the sandstone layers found in the McInnis Canyons National Conservation Area.

Entrada sandstone was formed between 180 and 140 million years ago as windblown sand dunes beside the Sundance Sea, which came to western North America as five separate intrusions of the Arctic Ocean.  今天, wind and water erosion are the sculptors that create rock art in the form of pinnacles, 拱门, 桥梁, 狭缝峡谷和壁龛与空中花园.

During a recent hike along the eastern rim of McDonald Canyon in the McInnis Canyons National Conservation area, I enjoyed the sinuous forms cut by ephemeral streams into large expanses of slickrock. These ended abruptly as dryfalls that become waterfalls after rains.

A particularly large and dramatic entrenched ephemeral stream is near the southern end of the McDonald Canyon. The stream cut deeper and deeper until the stream  banks were about 12 vertical feet.

靠近旱地, 河岸有六个开口, and each was stuffed with twigs of all sizes and finer debris.  Distinct drops of fluid, the color of tea, stained several of openings. Piles of plant debris and dribbles of dark fluid are certain evidence of a packrat or woodrat den.

六个开口, 横跨20英尺的河岸, 可能发现了多个不相连的洞穴, 但我有一种预感，它们都是一个巨大的, 蜿蜒的洞穴, 被一个背包客占据, 也可能是一只成年雌性在养育后代.

Packrat dens are common in canyon country, but this one occupied especially attractive sandstone. It has finely striated pink and white sedimentary layers and erosion has shaped the layers into steps and shelves.

On both sides of the opening to the den, water had fashioned labyrinthine columns. I smiled, thinking that this packrat had a refined sense of aesthetics.

The banks of the entrenched stream are in Entrada Sandstone, 而河床则有不同的颜色, 因为它是温盖特砂岩.

Packrats arose and diversified in the Pleistocene Epoch, from 2.600万到11700年前. 大约有21种属于该属 Neotoma, and four currently occupy the Colorado Plateau: bushy-tailed, desert, whitethroat and Mexican.

I suspect that this den was occupied by bushy-tailed packrats, Neotoma灰质，因为它们是最常见的. 它们的重量可以高达0.75 pounds and their total length is up to 16 inches, but almost half of that is tail. 

Packrats are herbivores, eating a wide diversity of things, even conifer needles. On the Colorado Plateau, they frequently inhabit crevices and caves eroded by water in sandstone. A den is usually 被一个背包客占据, or an adult female and her offspring. They make a warm nest of soft plant material and they also accumulate a pile of materials, 一个垃圾箱, 从他们的领土上收集来的.

他们在中间小便, 等到时机成熟, 尿液硬化或变硬, encasing the entire collection into a glassy amber mass called amberat. 隔绝氧气, 水和运动, these materials may be nicely preserved for up to 50,000年. The amberat, which is essentially crystallized urine, dissolves easily in water. 当水渗入洞穴时, 溶化的琥珀从洞口滴落下来, 在峡谷壁上留下琥珀色的痕迹.

因为包装鼠收集各种各样的材料, 比如松果, 种子, 叶子, 浆果, 骨骼和牙齿, middens have served as time capsules for ecologists and biogeographers. 当生物学家解剖昆虫时, they cut amberat disks an inch thick and a foot in diameter and wrap and label each one.

Each disk is carbon dated to determine when packrats collected the contents. Then they identify as many of the preserved contents as possible. This information can be used to describe where a species lived 25,1000年前, or where it migrated as glaciers moved across the landscape.

Evolutionary geneticists are now using midden materials to describe how a species changed over time, identifying which genes responded to climate change or evolved as the species migrated and adapted to new environments. 除了, viral and bacterial species can be identified and studied from ancient DNA in the amberat disks.

I enjoy finding packrat dens by looking for twigs and other debris spilling from openings and amber stains on canyon walls. These telltale signs serve as a reminder that although sandstone walls look solid, they may be riddled with passages formed by water and occupied by packrats peeing on time capsules.